Boring question #31: Why are all Cupids naked as tradition but they shell the breast of mermaids?

It always stuck me as funny that when ever I seen a Cupid on a card or an illustration he was always naked as he traditionally was. There was no diaper or lion cloth put upon him. When you see most mermaids, they fix shells over the breast. That is not how they traditionally started to my recollection. I have seen old illustrations and prints of mermaids and they were always topless. Why the shells? Where did that ”taking of liberties” get started?