a) participating in already existing, well-defined communities. Mastodon works for that, but so does Discord, Gemini, Gopher, tilda communities, IRC servers, and so on. Mastodon's peculiarities like tiny scale instances, inter-instance admin wars, non-existent search are perfectly fine for this use case. That's what it is made for, effectively.
b) connecting with friends and following their interests. This is just miserable with Mastodon: you might be on different instances that mute/ban each other, you might not find each other if you don't know their instances, if instances are interest-specific you have to follow on a number of instances, if their instance admin gets fed up with it and shuts it down you need to reconnect somehow, etc. Mastodon is not an identity-first design, it's community-first, and it's a problem for this use case.
c) following celebrities or creators famous in the world of atoms. Celebrities would want massive following, which requires massive scale, which is hard to achieve for something effectively run by volunteers. If it's smaller scale celebrities, they'd also want discoverability, which is hampered by the lack of search, federation bans, etc.
Mastodon is widely presented as a "twitter backup/alternative". It's just misleading unless the person moving to it only wants (a). There is nothing wrong with wanting (a)! However, it means that Mastodon has very little similarity to Twitter if people want (b) or (c).