Excerpts from the Oxford English Dictionary definition of costume:
2. a. The mode or fashion of personal attire and dress (including the way of wearing the hair, style of clothing and personal adornment) belonging to a particular nation, class, or period.
3. a. Fashion or style of dress appropriate to any occasion or season; hence, dress considered with regard to its fashion or style; garb.

So yes, the kilt is a costume. So are short pants and a t-shirt as appropriate to summer beach wear, trousers and a dress shirt and tie to the office, et cetera. But a kilt is always fashionable, always appropriate to the occasion or season, and so always a costume, right?