It's a bummer that some people feel the urge to flash in inappropriate public places. There are times and places for nearly everything. Really, the airplane is no different than the bus or the museum or the zoo (depending on which side of the fence one stands). Some public places allow for partial or full nudity. I know of a place where exhibitioning and voyeuring is a part of the make up of the domain.
Ultimately, the length of kilt or skirt is of slight relevance. However, the final point remains, Know your environment. Adhere to local rules. If you don't know, either "Observe and learn," or, "Ask."