A sufficiently powerful witch might be able to turn a frog into a prince, but it is unlikely the resulting creature would be getting kissed by many princesses. Why? Because when a witch turns a prince into a frog, he retains the intelligence of a prince. This implies that a frog who was enchanted into the form of prince would be likely to retain the intelligence as well as the habits of a typical frog. And most girls don't make a habit of kissing boys who eat flies.
So our hypothetical witch would probably be forced to get a little more creative. She could enchant a princess into a prince or a prince into a princess, but if it is a princess initiating the kiss as in most versions of the legend, this might not have such drastic consequences. Just because a princess sets out to kiss a prince, it does not always follow that she would have any strong objection to kissing a princess, and vice versa. So, the witch's little practical joke could fall flat quite easily if the girl is adept at rolling with the punches.
No, the thing for the witch to do is to transform a commoner into a prince. Most people who are at the social level where they can just go ahead and steal a kiss from royalty are not the sort to go around locking lips with the hoi palloi, at least not when they're not out slumming. A princess who believes herself to be kissing a genuine prince, only to find out he's really a member of the Great Unwashed, would probably take it even worse than if he did turn into a frog!