For Atlantis at least, you are seeing the benefit of ANIMATION over live action. It allows for more creative use of color and design without tying it to an actual human. As for high melanin low sunlight folks, much like the Inuit, perhaps these Atlanteans (and mermaids) are just VORACIOUS eaters of whale blubber and seals. Thats why Ariels friends are things you'd normally think she'd eat, like birds, crabs, and fish
"The Little Mermaid", as a fairy tale, has it's roots in Northern Europe and is CLEARLY written with a very specific demographic and owes pretty much all of its roots in Germanic folklore and myth. So to strip away some of it (like some of the characteristics of the mermaid herself "They were six beautiful children; but the youngest was the prettiest of them all; her skin was as clear and delicate as a rose-leaf, and her eyes as blue as the deepest sea; but, like all the others, she had no feet, and her body ended in a fish’s tail..... He fixed his coal-black eyes upon her so earnestly that she cast down her own, and then became aware that her fish’s tail was gone, and that she had as pretty a pair of white legs and tiny feet as any little maiden could have; but she had no clothes, so she wrapped herself in her long, thick hair. ....The slaves next performed some pretty fairy-like dances, to the sound of beautiful music. Then the little mermaid raised her lovely white arms, stood on the tips of her toes, and glided over the floor, and danced as no one yet had been able to dance."
http://hca.gilead.org.il/li_merma.html) but keep so many other aspects (including, apparently, the historical time period) it cultural appropriation without respect to the source. Now if they took the core of the story, especially the Christian message about gaining an immortal human soul, in an adaptation that removed the rest, then cast whomever you like. But as it is, this is like taking an African mythology, SETTING IT IN HISTORICAL AFRICA, but casting some random races in there 'cause reasons. Sure, it was totally done (John Wayne as Ghengis Kahn comes to mind) but that doesn't excuse it.
Disney KNOWS that folks love TLM because of the beauty of the story (especially their version of it) AND the aesthetics of the period in which it was set. Ships of sail, a prince, those aspects are key which is why they didn't just set it in modern american Miami or whatever. They want to imitate the cartoon as much as they can but virtue signal as well. It's lazy at best IMHO because it deprives us of A. an original mermaid tale based off ACTUAL African origin mythology with the weight of Disney behind it, B. is a clear cash grab playing off the legacy of the cartoon and is HIGHLY unlikely to give any additional value , and C. is (potentially) insulting to anyone who appreciates the original story in a cultural context.