There really aren’t a whole lot of people getting angry over this. Twitter simply acts as megaphone for fringe opinions. Remember that article from months ago about how “Progressives” only make up 8% of the US population? You wouldn’t think so by what goes on on social media.
The people getting angry about Ariel being black are mostly right-wing NPCs who have no idea why they are angry other than they’ve seen people they follow getting angry, so they assume this is the correct response. Most of the actual (white) racists are thrilled by this because they can use it to further their “white genocide” narrative.
I think more people are perturbed than feel comfortable admitting for fear of that sentiment being misconstrued as racism. We all know this wasn’t a colorblind casting. The actress is being used as a token to pander to woke crowds that are just going to find something else to complain about anyway. Where are the trans characters? Why can’t Ariel be a Muslim? Why are they a casting a human instead of a furry? I think you get my point.
Also, look at the current social climate. We’re in the middle of a culture war, social media is seeped in diversity-is-about-erasing-white-people clickbait, and this comes on the heels of Pride Month, so people are pretty disgusted with fake corporate wokeness anyway.