Haha, Yukko is my favorite character as well~ but the hitting is supposed to be comical, no one actually gets hurt.
In Japan there's this comedy routine where one person is the "idiot/bokke" and the other one is the "replier/tsukommi". The bokke says something dumb and the tsukomi replies by correcting him and then hitting him, with a paper fan, generally. Sounds weird, but it's cultural~
So, Yukko is a natural bokke and Mio, a born-tsukomi.