What's the problem with that, or what do you expect? GitHub mobile web truncates the README so you'd have to click twice (which always annoyed me about GitHub mobile web!).
I would expect it to show me the readme rendered, same as visiting it in a browser does. There didn’t appear to be a way to show it rendered at all either. Which is a massive problem if it’s going to hijack my browser so I cannot see it rendered at all from my mobile.