I think it's still under the tipping point. The more fans use it, the more it'll be good for fandoms, but it sometimes takes more fans onboarding and the thing they're a fan of also at least nominally present for it to match the numbers of other platforms.

But in terms of functionality for fandoms, it's as good as twitter was. It doesn't have the same purpose as tumblr or Facebook, so those can't be directly compared. I would say that in terms of what a fan base could use Mastodon for, it matches or exceeds Facebook simply by virtue of not being corporate, which means no artificial barricades. Tumblr is its own thing though, and it isn't going to be 1:1 with lemmy/piefed or Mastodon. The use case just isn't close enough.

There are fandoms present on Mastodon, mostly anime based, and the hashtag system works well for that, imo.