Make the mobile website work well instead. Diff view is unusable, as are many other views. I'd much rather have a working web version than an app for GitHub (and almost all other services). Their UI is all document retrieval and matches the web model really well.
diff on mobile? you mean adapting a UI based on 80ish monospace characters to a screen that fits in your pocket?
I don't think anything is impossible but it almost seems we would have to code differently for that to be viable.
Out of curiosity I've checked out terminal apps for both Android and IOS (Termux and ISH) and I like them quite a bit but would never use them for anything serious. Try them out and how you feel about using them for real stuff - for me, the enthusiasm dwindled and I felt like waiting until I had access to a desktop.
There's lots of room for improvement but they should focus on the more social aspects of Github like Issues and pull request discussion.