Why would a mermaid have that colour skin pigmentation when the ocean filters/reflects the UV rays of the sun
You're actually more likely to have a dark body if you're in the water and swimming free, unless you happen to be an apex predator or too big to be eaten easily by anything else.
There is far more to skin pigmentation than "UV protection", as albino animals tend to find out, skin colouration is an important survival aspect in wild animals. If you're a free swimming fish/animal in an ocean where there is any light then being a nice light colour is likely to result in you getting eaten, unless you're either an apex predator, or too big for the apex predators to worry you (or part of a swarm/school with so many others that are close together that your chance of survival as an individual goes up).
Hence why so many fish aren't in fact white...and why so many of the deep water fish are actually dark coloured, or indeed multicoloured (in dim light what stands out more, something dark/grey or white?).
I saw a comment from an author who wrote a story about "realistic" mermaids, apparently the marine biologists she spoke to when writing it made it very clear that a "light" colour mermaid of the "traditional little mermaid" type would be dinner for many of the predators (IIRC the author in question has a degree in parasitology and teaches it, which puts her on a fairly good level for understanding science, and she uses that to make a lot of her books more realistic).
Besides as has already been been said, it's a mermaid, you're already suspending disbelief in that this mythical creature exists in the first place, and can get legs by magic, so why does skin colour matter for "authenticity" (not to mention disney's Arial is not authentic to the original book, either in character or story).
Personally I don't expect to like this film, or ever watch it (it's not my cup of tea, and the other disney live action remakes have tended to be..bad), but then I also recognise I'm not a young child and not really the target audience of disney princess stories, in much the same way i'm not the target audience for a Hugh Grant romcon, or a Bollywood musical.