Mastodonians, Kbingers, and Fedizens are all superb. I’m not a fan of “feds” because it has another pretty common usage. I feel like whatever term is chosen should unambiguously evoke the fediverse.
The "fed" nickname was a bit facetious for exactly that reason. I just think it'd be funny for everyone to start calling eachother narcs. The internet is already full of that kind of fedjacketing, so why not embrace it?