Any DB is fine. Nostr content is just a text string. And some text meta data.

Depends on your use case really. mongo should be a decent choice. sqlite for small things.

Nostr should not be used as a database, but everyone does is. Because it's convenient. But you end up hitting walls. Like no real indexes, or query language, no ability to do large follow lists, AND/OR facilities, joins, secondary keys, composite indexes etc.

You might want to have a look at #ditto which is the first relay to have a modern database. Nostr as a DB is 50 year old technology.