I think a lot of people have an issue with people wearing a Kilt on St. Patrick's day because there are so many people now who just think 'The Kilt was Irish first.' I know myself that I bridle at some of the pub historians who tell me the history of everything Scottish which is actually Irish.
I certainly believe that the Kilt has come to represent a lot of 'Celtic' cultures because they have adopted it.
I think that's great. But I do like people to know the proper history of things and are willing to learn more when they are wrong. (I'd be an idiot if I hadn't learned that lesson.

So I would say go ahead and wear the Kilt if you want to, but wear it with the knowledge that some people don't know the history of the Kilt and may make incorrect assumptions. (And will, sometimes, REFUSE to admit they are wrong.)

I will be wearing my Kilt on St. Patrick's day because I wear my kilt every day.
That said, I won't be celebrating St. Patrick's day.