An excellent thread depicts a lot of AI created imagery, much of which is bizarre. I hesitate to call it “art”, but if it is, why hold people to a higher standard?
Breasts are needed to provide nutrients to baby mermaids. If they are spawned from fish eggs, evolutionary embryology suggests a high initial rate of growth.
If mermaids are mammals, and they live birth in water, they’d have to deliver feet first, so the babies don’t drown.
And mermaids should really have blowholes, or at least dorsal oriented nostrils.
Some sharks give live birth. As well as male seahorses.
It is well known how some sharks and seahorses birth. Our knowledge of mermaids remains woefully inadequate.
Somewhat randomly, this article in public art appeared today. You think an airport might not be the best place for scary works…
I remember that horse! The sculptor (Luis Jimenez) died in an accident while creating it. His daughter was a contestant on Project Runway – that’s where I first heard of him and this statue.
No, but I have seen reptiles with wings. In addition to four legs.
And it’s not too hard to see how to modify the basic vertebrate body plan for more legs, either. As it is, both of the places where vertebrate legs attach, the spine continues both before and behind that point. OK, so put in a pair of shoulder blades, then more spine, and then another pair of shoulder blades. To put the butt behind the tail, though, you’d need (at least) a digestive tract that twists around the spine.
I don’t think mermaids are mammalian. I think that the “breasts” are purely a mimickry feature, to make it easier for them to attract their prey.
That, I think, is just an artifact of the artists working from human models.
Our swift approach the Siren choir decries
Celestial music warbles from their tongue
And thus the sweet deluders tune the song:
“O stay! O pride of Greece! Ulysses! Stay!
O cease thy course and listen to our lay!”
I doubt many mermaid artists are half as worried about “real vertebrae biology” in their pretend fish-girls as some art viewers.
Woody Allen (a long time ago) mentioned a story where the mermaid was half and half, split down the middle.
I’m a leg guy, so I’d happily marry an inverted mermaid, provided she has shapely human legs. I’d like her fish half to be a mudskipper with modified pectoral fins allowing her to crawl out of the mud at night so we could cuddle. That sounds romantic. 
Who knows, maybe my li’l cutie and I would spawn a big family of little fry…or if things don’t work out, I’d have a big fish fry. That sounds tasty. 
Are you the keeper of the Eddystone light?
“Why can’t she be the other kind of mermaid, with a fish for a head and the lady parts on the bottom?”
/Fry
ETA: I think the statue is fine.