It has nothing to do with being a black mermaid, the problem is being a black mermaid replacing the original
Ariel's original identity IS the cartoon, because the cartoon IS the original Ariel.
Ask any 3 year old to point to Ariel, they will point to this pale colored red headed figure:
Ask any 3 year old if this is Ariel:
Any they will look at you dumbfounded. She is a mermaid, she can be a Disney character. But she is not the established character Ariel and she does not share the Characters story, history, and identity.
If disney wants to write a story about a black mermaid and cast Halle Berry to play her, by all means. But write a new original story about this new original character.
How this not blackwashing AND cultural appropriation is beyond me.