... Honestly, I think Rose isn't actually being particularly stupid right now. ^^; If the Headmaster didn't think it was totally okay, she'd be being stupid, but at the moment she's more being silly then anything.
I mean, when a Witch is weak and not a threat at all, isn't treating them nicely a lot better then attempting to kill them horribly? If they do become powerful at some point, then they'll have lots of memories of being treated nicely and pleasantly. That's probably not the chief matter in Rose's mind(she's more concerned with hugs and ice cream) but it's there somewhere.
I just think it's kind of weird to see her called a complete idiot for something the Headmaster basically said was just fine.
... Not that she won't do anything stupid, she's totally oblivious and a complete ditz even if she's not actually dumb so it's going to happen plenty, but this time it's just a little weird.