I don’t have an account there.
My discover feed in a browser with English was some generic memes and jokes, something about Drake’s music, some comments on netflix and movies, etc.
My discover feed in a browser with Japanese set as the main language was some of the same, photography, anime drawings, and probably about half of the literal same posts (in English) as above. This could, I guess, be related to other sites I have visited or cookies or something.
Anecdotally, a lot moved to Insta or maintain both xitter and insta, but yeah, there’s no real inertia to move and, outside of otaku circles, it seems that mastodon and the fediverse are basically unknown.