I personally don't think that mermaids would have human norms of modesty. It looks logical to me that they don't cover their breasts with any tops. I like image of mermaids who cover their chest with long hair, but shell/starfish bras (especially mermaids in modern tops) seem a bit strange in my opinion. It is a way of censorship, which makes mermaids "family-friendly", but also makes them more human-like and less mysterious.