Is it safe to upload JSON files to upload folder?

JSON files are really data files: they are not a security threat per se, it's simply that WordPress doesn't include them in its whitelist.

It really depends on who can upload the data and what you do with the data. After all, an Excel file with macros can be a security risk, and yet WordPress allows the following:

'xla|xls|xlt|xlw' => 'application/vnd.ms-excel',
'xlsx'            => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
'xlsm'            => 'application/vnd.ms-excel.sheet.macroEnabled.12',
'xlsb'            => 'application/vnd.ms-excel.sheet.binary.macroEnabled.12',
'xltx'            => 'application/vnd.openxmlformats-officedocument.spreadsheetml.template',
'xltm'            => 'application/vnd.ms-excel.template.macroEnabled.12',
'xlam'            => 'application/vnd.ms-excel.addin.macroEnabled.12',

Now that we've established that what is considered "safe" depends on perspectives, it's undeniable that whitelisting is more secure than blacklisting, but sometimes false positives may occur (there are sooo many file types, after all).

Here is the full list of allowed file types by default:

/**
     * Filters the list of mime types and file extensions.
     *
     * This filter should be used to add, not remove, mime types. To remove
     * mime types, use the {@see 'upload_mimes'} filter.
     *
     * @since 3.5.0
     *
     * @param string[] $wp_get_mime_types Mime types keyed by the file extension regex
     *                                 corresponding to those types.
     */
    apply_filters(
        'mime_types',
        array(
            // Image formats.
            'jpg|jpeg|jpe'                 => 'image/jpeg',
            'gif'                          => 'image/gif',
            'png'                          => 'image/png',
            'bmp'                          => 'image/bmp',
            'tiff|tif'                     => 'image/tiff',
            'webp'                         => 'image/webp',
            'ico'                          => 'image/x-icon',
            'heic'                         => 'image/heic',
            // Video formats.
            'asf|asx'                      => 'video/x-ms-asf',
            'wmv'                          => 'video/x-ms-wmv',
            'wmx'                          => 'video/x-ms-wmx',
            'wm'                           => 'video/x-ms-wm',
            'avi'                          => 'video/avi',
            'divx'                         => 'video/divx',
            'flv'                          => 'video/x-flv',
            'mov|qt'                       => 'video/quicktime',
            'mpeg|mpg|mpe'                 => 'video/mpeg',
            'mp4|m4v'                      => 'video/mp4',
            'ogv'                          => 'video/ogg',
            'webm'                         => 'video/webm',
            'mkv'                          => 'video/x-matroska',
            '3gp|3gpp'                     => 'video/3gpp',  // Can also be audio.
            '3g2|3gp2'                     => 'video/3gpp2', // Can also be audio.
            // Text formats.
            'txt|asc|c|cc|h|srt'           => 'text/plain',
            'csv'                          => 'text/csv',
            'tsv'                          => 'text/tab-separated-values',
            'ics'                          => 'text/calendar',
            'rtx'                          => 'text/richtext',
            'css'                          => 'text/css',
            'htm|html'                     => 'text/html',
            'vtt'                          => 'text/vtt',
            'dfxp'                         => 'application/ttaf+xml',
            // Audio formats.
            'mp3|m4a|m4b'                  => 'audio/mpeg',
            'aac'                          => 'audio/aac',
            'ra|ram'                       => 'audio/x-realaudio',
            'wav'                          => 'audio/wav',
            'ogg|oga'                      => 'audio/ogg',
            'flac'                         => 'audio/flac',
            'mid|midi'                     => 'audio/midi',
            'wma'                          => 'audio/x-ms-wma',
            'wax'                          => 'audio/x-ms-wax',
            'mka'                          => 'audio/x-matroska',
            // Misc application formats.
            'rtf'                          => 'application/rtf',
            'js'                           => 'application/javascript',
            'pdf'                          => 'application/pdf',
            'swf'                          => 'application/x-shockwave-flash',
            'class'                        => 'application/java',
            'tar'                          => 'application/x-tar',
            'zip'                          => 'application/zip',
            'gz|gzip'                      => 'application/x-gzip',
            'rar'                          => 'application/rar',
            '7z'                           => 'application/x-7z-compressed',
            'exe'                          => 'application/x-msdownload',
            'psd'                          => 'application/octet-stream',
            'xcf'                          => 'application/octet-stream',
            // MS Office formats.
            'doc'                          => 'application/msword',
            'pot|pps|ppt'                  => 'application/vnd.ms-powerpoint',
            'wri'                          => 'application/vnd.ms-write',
            'xla|xls|xlt|xlw'              => 'application/vnd.ms-excel',
            'mdb'                          => 'application/vnd.ms-access',
            'mpp'                          => 'application/vnd.ms-project',
            'docx'                         => 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
            'docm'                         => 'application/vnd.ms-word.document.macroEnabled.12',
            'dotx'                         => 'application/vnd.openxmlformats-officedocument.wordprocessingml.template',
            'dotm'                         => 'application/vnd.ms-word.template.macroEnabled.12',
            'xlsx'                         => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
            'xlsm'                         => 'application/vnd.ms-excel.sheet.macroEnabled.12',
            'xlsb'                         => 'application/vnd.ms-excel.sheet.binary.macroEnabled.12',
            'xltx'                         => 'application/vnd.openxmlformats-officedocument.spreadsheetml.template',
            'xltm'                         => 'application/vnd.ms-excel.template.macroEnabled.12',
            'xlam'                         => 'application/vnd.ms-excel.addin.macroEnabled.12',
            'pptx'                         => 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
            'pptm'                         => 'application/vnd.ms-powerpoint.presentation.macroEnabled.12',
            'ppsx'                         => 'application/vnd.openxmlformats-officedocument.presentationml.slideshow',
            'ppsm'                         => 'application/vnd.ms-powerpoint.slideshow.macroEnabled.12',
            'potx'                         => 'application/vnd.openxmlformats-officedocument.presentationml.template',
            'potm'                         => 'application/vnd.ms-powerpoint.template.macroEnabled.12',
            'ppam'                         => 'application/vnd.ms-powerpoint.addin.macroEnabled.12',
            'sldx'                         => 'application/vnd.openxmlformats-officedocument.presentationml.slide',
            'sldm'                         => 'application/vnd.ms-powerpoint.slide.macroEnabled.12',
            'onetoc|onetoc2|onetmp|onepkg' => 'application/onenote',
            'oxps'                         => 'application/oxps',
            'xps'                          => 'application/vnd.ms-xpsdocument',
            // OpenOffice formats.
            'odt'                          => 'application/vnd.oasis.opendocument.text',
            'odp'                          => 'application/vnd.oasis.opendocument.presentation',
            'ods'                          => 'application/vnd.oasis.opendocument.spreadsheet',
            'odg'                          => 'application/vnd.oasis.opendocument.graphics',
            'odc'                          => 'application/vnd.oasis.opendocument.chart',
            'odb'                          => 'application/vnd.oasis.opendocument.database',
            'odf'                          => 'application/vnd.oasis.opendocument.formula',
            // WordPerfect formats.
            'wp|wpd'                       => 'application/wordperfect',
            // iWork formats.
            'key'                          => 'application/vnd.apple.keynote',
            'numbers'                      => 'application/vnd.apple.numbers',
            'pages'                        => 'application/vnd.apple.pages',
        )
    );

This is where the mime_types filter comes into play ; you can place the following in your theme's functions.php or in a custom plugin:

add_filter('mime_types', 'add_json_mime_type', 10, 1);
function add_json_mime_type($mime_types) {
  $mime_types['json'] = 'application/json';
  
  return $mime_types;
}