Hm? Cirno wears dresses all the time. She doesn't like wearing pants, because they feel too tight and restricting on her. She doesn't like wearing short skirts, because they are too revealing.
That's too bad...

I'll have to admit, skirts probably are less restricting... There is a scene in a book I just read where a group of girls are discussing whether dresses would be good for fighting in and one of them says she thinks they would allow much greater freedom of movement, and who cares if you flash your underwear at somebody if you're kicking their ass? (Then one of the guys comes up and says "Somebody's flashing underwear? Alright! I'm in!")