I don't see Mastodon getting worse by getting larger.
You only see who you follow, and there's no like/karma/upvotes algorithm.
Everything is sorted chronologically, and if anyone tries to "game" that by posting too much, they'd get unfollowed and/or banned.
Mastodon is a nurtured cultivated twitter.
Personally I follow the CSS/JS/TS community (for work), the gamedev community (for fun), and the space community (for passion)