Well, actually, PieFed.social has enabled it’s API which means that the app Interstellar can be used with it. Find out more about the app at https://interstellar.jwr.one/. iOS users can use the testing version at https://testflight.apple.com/join/JXRtHye2
https://feddit.online, another PF instance, has enabled the API too and I expect others will follow in good time.
Does it work ok in your phone web browser?
Yes, I can browse Piefed instances on my phone web browser and log without issues.