I have a theory as to why Sock wears a skirt.
Sock has a middle name Maxwell that comes from the Scottish steam that was called "Maccus Wael", and as we all remember, Scottish people wear kilts. So, I guess Sock just feels kinship with his distant relatives and shows it by wearing a skirt, but he doesn't really care if it's a plaid kilt or a plain skirt with jeans underneath. No one gets it and he either has to explain it to everyone or ignore endless questions. Did I get to the truth?