If you really need it to be a proper mobile app then it's unlikely you'll find one - this would be quite a lot of work as every platform has implemented it's own API for talking with client apps, rather than using ActivityPub. The app would need to talk to several different APIs.

You're better off choosing a platform that can talk with as many different types of other platforms as possible. And when I say 'talk with' there will be gradients of talking with differing amounts of problems. For example PieFed is great with Lemmy, good with PeerTube and Ok with Mastodon. Then use the client app for your chosen platform.

Choosing the platform will be a process of just trying them all. Check out friendica, they cast a wide net.