Because she isn't "the mermaid." She's Ariel.
And this is what Ariel looks like:
To take your Lando example imagine if Disney decided to make him white in a new SW film. Lando Calrissian. Not "the administrator of Cloud City which doesn't exist so who cares what his race is" but *Lando Calrissian.* Yes he's a fictional character that lives in a non-existent galaxy. Yes Ariel is fictional too. But just because her character isn't real doesn't mean that she doesn't have a pre-defined look. Not even just a pre-defined look, an absolutely iconic one. White redheaded girl with a fish tail. It would be just as stupid if they made her a man or gave her a propellor tail instead of one from a fish. She has an established look and Disney is deviating from it for the most laughable of reasons.
If they wanted to pull a "Lando Calrissian" then they could have invented a brand new character just like George did. He didn't suddenly make Luke Skywalker black in order to virtue signal to slaves of identity politics.