Apps are in development. I'm testing it right now. Writing this from Piefed in Interstellar, which recently started supporting Piefed.
I'm also testing Pferd, developed by the Piefed devs.
Both are very early in development, some bugs here and there, but I like that there are different options already. I could see myself switching to Piefed when apps are stable.
The ideal situation would be if my favorite Lemmy app Voyager would start to support Piefed, but I don't know if that's a possibility.
This is something I'd like to investigate, but will probably happen after lemmy 1.0 API changes are stable/released. I'll keep tabs on progress though!
@freamon@preferred.social is the one primarily working on the api, pinging them to bring this to their attention. There is also a matrix chat for piefed dev where I have seen api discussions happen.
My understanding as a non-dev is that the api is 95+% mostly the same as lemmy (intentionally so). The rest is mostly there, but with some kinks still to work out. Right now, preferred.social is the only piefed instance running a build with the api turned on for testing.
I'll just say, even if 95% of the API is the same (which I am almost sure it's less than that), it will still need very intentional effort in order to implement the support. Interstellar already had support for Lemmy, but it took us several weeks to get to the point we are now with PieFed support, and that's all with a code architecture that makes it easy to add new platforms to support (due to both Mbin and Lemmy support previously).
All that to say, it likely will not be as easy as it seems to add PieFed support.