It's awesome that you want to wear your kilt to prom. While I do agree that the prom isn't just for the ladies, keep in mind that it's not just for you, either. I think you mentioned that prom isn't a "big thing" for her so you may not have to worry about smashing her dreams, but still... You still have to keep in mind her opinion. If she just doesn't like the kilt but doesn't hate it either, you might be ok. But if she finds it ridiculous, she may feel embarrassed to be with you at prom. You definitely don't want that. Of course you'll have to feel that one out... even if you manage to get her to say it's ok with her, keep in mind that doesn't mean she won't begrudge you or it

Another thing to keep in mind, especially when deciding on whether you're going to ask permission or just show up unannounced. If you are denied entrance because of the kilt, she could be all dressed up with no place to go, as well. Your decision can dramatically affect her evening. So... be prepared. Either have a back-up outfit you can change into, or back-up plans you can fall back on (assuming she really, truly, doesn't mind not going to prom).

In short... I'd say if she has issues with the kilt and she means more to you than just "some girl" you asked to prom, take her feelings into account and wear something more... well... boring Even if you win the battle of her agreement, you could still leave her unhappy. And if she's unhappy... you're unhappy