You mean like one app to rule them all, something that can access all corners of the Fediverse? That would take an awful lot of work, but there's already Fedilab, which can access multiple accounts on Mastodon, Peertube, and Pleroma. I don't know if they intend to integrate more though.

https://fedilab.app/

there ... already are multiple ones?

I doubt it. While the server to server communication is standardized with ActivityPub, the server to client (app) communication isn't. This means different Fediverse apps have different APIs to login, fetch and create posts and so on. It's unlikely that we'll ever have a standard API here, because the apps work diffently and have different needs for their APIs. Maybe some kind of common denominator, but that probably wouldn't be feature complete on most apps.

So the one app to rule them all would have to created custom API Implementations and UIs for every Fediverse app, which would be a loooot of work and probably a worse experience than specialized apps.

So ... either a somewhat uninformed question (which is totally fine BTW, this is all new for everyone!) or a deep question!

1. To the former ... there are plenty of fediverse apps. They're all mostly specific to a platform such as lemmy or mastodon. Most or nearly all of the lemmy apps are still in beta can still be used if you signup appropriately. I think memmy came out with their official version just recently.
2. To the latter ... yea ... if the fediverse is a thing then why aren't there "fediverse" apps that aren't specific to any one platform but instead allow you to "surf the fediverse" in the same way that the browser lets you surf the web or an email app lets you use multiple email accounts from multiple providers? And ... that's a good question! There should be one IMO! It'd be somewhat tricky to make though and this whole fediverse thing is arguably still a prototype. But, IMO, it's gotten to the point now where this actually makes a lot of sense and it's quite pertinent to pose the question. Arguably, until it's possible to consume and browse the whole fediverse through one single interface, it isn't really a fediverse but something lesser.

Try out Liftoff if you're on Android. People also seem to like Jerboa. Both apps support all instances of Lemmy AFAIK

I'm using Memmy for Lemmy on IOS which is heavily inspired by be Apollo App and just recently popped up. So far so good. Still missing some features but it's already a better experience than the reddit official app