I think it’s a mixture of certain traits; for example, bunnies should be cute and short, have bunny-like facial features, and have the appropriate ears and tail. In Mei Han’s case, since she’s a mutant between a bunny and a cheetah (?), she’s too elegant and tall and has unique facial features that aren’t specific to either bunny or cheetah — it’s a mix.
You can also see it with the ML’s eyes — if we compare him to other foxes, you can see he doesn’t have the typical fox eyes that we’ve seen with a couple characters already, which is telling that he’s a mutant. This might all be completely wrong lmao, but from reading it, I think that’s how they wanted it to work ╮( ̄▽ ̄)╭

what exactly about her appearence reveals that she's a mutant cuz i don't see it?