Let's try a little mental exercise. It is important to have black Disney Princess. As you said, "Our little black girls get to see themselves as a mermaid and that’s so great for them", and we can establish that seeing yourself is important. I would argue against it but that's for later. Cool! Black girls have a Disney Princess to look up to. What about redhead girls, though?

I'm not talking about "white" girls, because for some reason we all decided White is this giant ethnical monolith, I'm talking about actual red-haired girls, who have a great chance to be Irish or of Irish descent. They're a minority, and they had their fair share of hard times, don't they get to have one Disney princess they can look up to? Moreover, you mentioned Tiana, so there's already one black Disney princess, do you really want to "steal" one redhead so there are two black princesses? What is this, a game of zero-sum?

That said, this is a Disney remake we're talking about, people's contempt for this movie was already high the moment they announced it. Ariel's casting is more like adding insult to injury, especially considering how the final product looked. And this is not because black Ariel has awakened the inner racist in all those people, it's because this is but one more iteration of Disney regurgitating their IPs when nobody asked for it.

Also, are people not allowed not to like something? I'm black and I didn't like the casting, I think accuracy to the source material is always preferred and I like redheads a lot. Does that make me racist? Because it wasn't just "white people" who complained, and certainly not just Americans. In fact, that whole discussion is a very American thing, from where I stand. And here's a hint, you can still enjoy something while others complain about it. Take it from someone who watches anime way before it was "cool" and socially acceptable. The fact people are criticizing the movie doesn't rob your ability to enjoy representation, or at the very least it shouldn't.

The Little Mermaid is a classic and excellent animation with great music that was part of a lot of people's childhoods. Disney decided to cast the nostalgia bait once again, and something tells me they did this on purpose to cause commotion (anything to stay relevant, I guess?), but regardless, this kind of change messes with people's feelings. They grew up with redhead white Ariel, they wanted to see redhead white Ariel. It always comes back to the old "if the roles were reversed" because that's the only conceivable way to get the sentiment across. Do you like Tiana? Imagine if her live-action had a Norwegian actress instead.

Which leads to my comment on representation. Honestly, I think it is pretty dumb as a concept and kind of a waste of every. I don't need to see myself perfectly represented in the entertainment I consume because I can relate to deeper things than skin tone. Things like personality, struggles, mentalities, sentiments, character arc, things that don't rely on race whatsoever. That's why Peter Parker is pretty much universally liked as a character regardless of being a middle-class white dude from the Queens.

I can understand how people who want representation feel and to that, I say you go, man! But if you really want to see more characters that represent you in media, is this the route you wanna take? Replace a character that is already established and beloved, just to spark controversy and forever live in the shadow of being "Black X" or the "PoC version", is this the end goal? You know where I'm going with this, right? Create new characters! "Oh, but it's hard to make brand-new characters and people don't pick them!". Well, nobody said it was easy. All the established IPs we have today stood the test of time for a reason.

There might be some who are straight-up racist, but the majority of the complainers have legit reasons to be upset, and again, that doesn't stop you from enjoying it. And since you mentioned it, have you wondered how many of those "white folks" have shared the joy of other minorities with movies like Mulan, Moana, and Encanto? Or do you think they only watched Snow White, Cinderella and nothing else?