You are what you post
It’s Meta that would kill the fediverse.
Read: How to Kill a Decentralised Network (such as the Fediverse)
To get ui/ux suited for Mastodon, the app shouldn’t use e. g. the same interface for Lemmy.
Then the situation is almost the same as using different apps.
Oops. I always wonder how to recognise federated services as domain names do not say much. How does Voyager avoid that issue?