She doesn't look like the mermaid in the tale, which was made clear that she got "fair skin and blue eyes".
Remember, The Disney version is an adaptation of a public domain work meaning anyone can do what they want with those characters mentioned in that story (as they have for many years).
The mermaid also dies in the first original tale but Disney took liberties to alter that as they saw fit. In fact, that altered a lot. They placed a lot of American sensibilities into a Danish work that, although had a valuable lesson, was pretty bleak.
Several Disney fans don't even acknowledge Hans' version and think the Disney version is the original. Even if you tell them otherwise. They will say they want the original and question why was this or that changed from the cartoon, completely throwing the book under the bus, but they aren't ready for the real story, dumped and depressed Ariel drying up into seafoam on the shore.
So Disney's version is not the "original" version, it is an adaptation THEY created and it is THEIR work they trademarked, which means THEY can do whatever they want with any character in their extensive library.
This is what happened with Winnie the Pooh. Though people were crying about it, Winnie was made into a mess of a slasher/killer horror film. However, the makers of that film could do that since the original Winnie the Pooh story was in the public domain. Anyone can still do what they want with the original characters. However, they can not use the same Winnie that Disney modified for their own films as it is trademarked by Disney and Disney will sue them faster than Silent Glaive Surprise can take out the Earth.
However, if Disney one day decided, (though they probably never will) that Winnie should be a white Polar Bear, they could do that.
If they want a black princess, they are free to make one. Why insert it into existing story?
Lots of creators insert things into existing stories because they can. Manga Sailor Moon can make people think of much older works like a powered up Cinderella wearing Romeo and Juliet bracelets...
In the anime, Doom Tree arc is very similar to Adam and Eve from the Garden of Eden story in religious literature. There's even an episode in that arc in which the characters emulate a Snow White play.
Once again, it's important to note that these adapted characters belong to Disney. They are copyrighted and trademarked to Disney, so they can change it anyway they like. It is really is their story. Also, the actress was chosen because of her talent and skill for the role.
While I have seen people object to her I haven't seen them offer not one alternative that could beat her vocally for the role. They simply don't want her because of the skin color which is kinda weird because Ariel's skin doesn't make her unique, it's her voice. Ursula wasn't trying to steal her complexion...
The most important thing to remember is, Her skin color is irrelevant to Disney because it's not a factor in their version of the story. Not only that, this isn't a remake of the Little Mermaid, it's a reimagining. It doesn't invalidate the original cartoon version in anyway nor does it destroy the very first tale legacy either.
I think the issue with Little Mermaid was because she is widely recognized for being a redhead... If she was just another blonde princess, nobody would mind a unfaithful casting for a live-action version.
It's definitely mostly cause she's of African descent ("Black"). While I have seen people make mention of her hair color which is similar to Mako's almost btw. There are MANY people who have brought her skin color to the forefront. It's the main argument.
When they see how talented the actress is they start attacking other aspects of the movie, finding other reasons to invalidate it, or giving backhanded compliments.
"This movie looks too dark? Why is this ONE specific scene so dark?" (Scene takes place during a storm or when Ariel is in a darker part of the ocean like Ursula's liar.)
"I'm going just for Ursula. She is going to save the movie." (Intentionally not acknowledging Ariel, the star of the movie, played by a person with African features, but shifting everything to the villain played by a white woman reinstating they will never accept "that kind" of Ariel.) This isa common way some racist people show hostility towards darker skin people, they don't acknowledge their worth or try to make them feel uncomfortable by staring like they are looking at some kind of unknown creature.
"Disney is pandering to Black people."
Disney "panders" to everyone. Every story they've adapted has been watered down and altered in some way, including those Marvel films.
"Disney is too woke now..."
Disney started off quite a racist company, like many other business in the early part of the century. They are more inclusive now even though they still do make goofy decisions sometimes. I think an issue with SOME people is that they don't want to be reminded that times have changed and what was once held in a dark corner is not so much anymore.
It's usually the older racist people who gripe but these same people teach that same stupid knowledge about skin color to their offspring and the cycle continues and they feel like their skin color grants them some sort of superior status.
I don't even need to mention Namor being "too dark" for Latin America.
Sailor Moon needs to do a quick Moon Healing Escalation for Colorism across the globe.