It's a movie about mermaids and talking fish why should it matter? And if we're talking about staying accurate to the original work, Hans Christian Anderson's story is vastly different from the movie. While it's setting is never specified, it's theorized to either take place in Denmark or Italy, which aren't exactly known for their redheads. The little mermaid is only described as "the prettiest of them all; her skin was as clear and delicate as a rose-leaf,and her eyes as blue as the deepest sea; but, like all the others, she had no feet" So it's annoying that the actress doesn't have blue contacts, but at the same time nothing else in these movies are accurate, so why should her appearance be?

All in all, black/whitewashing for a political agenda is an issue, and Disney probably did do it for brownie points, but we have bigger problems then a fictional character's race changing when we have much bigger issues in the world to worry about.