You can post on as many NOSTR relays as you want. Get banned on one, you are still publishing on the others.
You can also, along with your messages, suggest to "subscribers" your preferred relays, so people that really want to follow you as you get banned all over the net always know the best place to reach you. This process of course would be automated by the clients.
The point is that you have only one Mastodon API endpoint vs potentially infinite Nostr relays for each account you own.
The point is you don't have that issue with nostr
This seems like a good area to improve since things like code signing could also benefit. One of the big problems I see is bootstrapping your network initially and also solving the lost phone problem without setting up a system which phishers and spammers would have a field day with.
Mastodon is clearly inferior to Nostr when it comes to having a decentralized identity.