I'll tell you what the least wonderful thing about mermaids is. It's that they have the magical ability to always have their hair positioned perfectly to cover their nipples.
Did you know that the whole mermaid thing came from sailors fucking dugongs? And that dugongs have the genitals most like human genitals in the animal kingdom?
Have I ruined mermaids, yet?
How about...mermaids poop outta their face?
Short Answer: That last one isn't true. Oh wait, mermaids aren't true, so they do poop outta their stupid, nipple-covered faces! Shut-up, mermaids! Nobody wants fish parts instead of lady parts.