the problem with casting a black actress as Ariel is that the movie is trying to send 2 opposite messages
The whole idea behind The Little Mermaid is that she falls in love with a human and is willing to completely change her body in order to romantically hook up with him. She has no pride in her fin or her mermaid body. To her it's just totally disposable in her quest for love.
So then Disney comes and makes another version of The Little Mermaid and they cast a black actress. And that's not a coincidence. That casting is intentionally meaningful. In an interview with Halle Bailey she said her grandparents told her that she doesn't "understand what this is doing for us, for our community, for all the little Black and brown girls who are going to see themselves in you". So this movie is supposed to be all about instilling pride in little black girls.
But that message about racial pride completely obscures the message already inherent in the story about the disposability of your natural bodily form. The basic message of the story is that you shouldn't take pride in your natural bodily form. Disney's new message is that much more distorted by Bailey's red hair in the movie. How many little black girls have naturally red straight hair? Are they really supposed to identify with a straight red haired mermaid?
They could change the race of Prince Eric and it wouldn't be a problem. The story doesn't involve him making any changes to his body.
The Little Mermaid is all about what happens below Ariel's waist line but the casting for this film just focuses the audience's attention on what's above her waist line and that's clearly a cinematic failure in its own right. There's nothing wrong by itself with Disney and Halle Bailey wanting her to be a role model to young black girls but it really has no place in this particular movie. Trying to shoehorn that message into The Little Mermaid of all stories is just obnoxious.