I think the best use case for Nostr is as a text-based Twitter-like decentralized and censorship resistant social media platform.

The issue with Mastodon is that Mastodon, using the ActivityPub protocol, is not censorship resistant and that your identity is tied to one instance.

There are some ActivityPub instances which do happen to have permissive policies, however, although that is not the same as censorship resistance. An example of that is minds.com (which is not Mastodon-based, but does support ActivityPub. Their Nostr support, on the other hand, is broken), but there are also free speech instances of more common software (Mastodon, Pleroma and the like).