Fat isn’t a defense against water pressure, it’s an insulator. So the colder the water, the fatter the mermaid.
Scandinavian merfolk would be tubby, evolving more for persistence hunting in the freezing coastal waters, possibly relying more on siren-song to lull prey into effective range. The less moving the better! Gotta keep the chub on to survive the winter.
Whereas Mediterranean/Polynesian merfolk would spend their time among the fertile reefs in warmer waters, competing with other predators. They would be built for speed and agility, buff like sprinters. Darker skin would be harder to spot under the surf, and more resistant to the glaring sun.
Also: it’s not hair, it’s scillia that they use to filter plankton out of the water, and it stings like a jellyfish if you touch it.
I would imagine that merfolk come in many different colors, maybe they even have the ability to shift their skin tone to assist in hunting? I guess that depends on how intelligent your mermaids are.
Tldr; cold water mermaids would probably be chubby, warm water mermaids would look like Serna Williams.