Why is he such a prick about her acting like a cat
He's mostly over the whole act like a cat/act like a human thing, actually - he wasn't acting like a prick with her about the dog, until the way she was clinging to him made him embarrassed and he got a bit snarky. And to be honest, he wasn't being all that much of a prick with her even then.
It's pretty obvious that the more he actually gets to know her the more understanding and accepting he is of her little quirks. He's not quite there yet, but he's clearly on the right path, and it's definitely been helped by Mi-chan actually backing down and just trying to be a friend. They've both got some bad habits to work through, but there's no way you can claim there hasn't been a whole lot of progress.