Most Helpful Guy
Ah yes with the well placed long hair - I suppose human clothes would bring them closer to our world so by having them naked while covered makes them seem mysterious without raising eyebrows.
Most Helpful Girl
Mermaids: clam bras aren't very realistic and I doubt they would have clothing.
Faeries: I'm not sure, I'm assuming people can't decide what a faerie would wear.
Angels: old depictions probably.