Uhm, this might be a little insensitive, but why not just write comments? That would be understandable by everybody, including blind people who would be excluded with sign language videos.
This sounds to me as convenient as making a comment as an audio recording. Ie not at all.
That said, video capabilities are constantly improving. But as most instances are run and financed by volunteers, adding video uploads would greatly increase the disk space requirements. So I guess you would need a specialised instance to host the videos that would have to be policed so that the feature wouldn’t be abused to host illegal videos or porn.