Choosing one of the wishes he thought up, Jon said "I wish that Karyn was the hottest girl in school." Karyn may have thought that her wish for larger breasts and blond hair was a bad thing, but after looking at the new Karyn, Jon figured that it wasn't. So why not go all the way and make her the hottest girl around? The way Jon saw it, he was doing her a favor. It's not like he was making her ugly or something.
Unfortunately, Jon didn't realize that Karyn and everyone else would notice the result of his wish, since this was actually The Obvious Stone. And some people may not like that Karyn was now the hottest girl.
