A Mastodon instance is more like a Nostr relay except you have to sign up with an email and password, the admin chooses what other instances you're allowed to see posts from, and since your account exists only on that single relay, if it goes down your account disappears.

Not very decentralised and (by design) not censorship resistant because the creator of Mastodon loves censorship.

That's why the Nostr <> fediverse bridge was made by the Soapbox dev.