This is why I am sometimes glad I come to these comic movies and tv shows without really knowing the comics. I don't know whether Iris West should be black or white based on the comics so it doesn't bother me.
I am somewhat familiar with at least some comics, but it still doesn't bother me if an adaptation changes anything, because it is an adaptation, after all. It's not trying to copy or replace the comics, but to create a new work inspired by them. The differences between the two versions of the concept can be intriguing in their own right.
I mean, really, if I were a purist about what comic-book characters looked like, then wouldn't I have to object to them being flesh and blood rather than ink and dye? I mean, they don't actually look like living people, they look like drawings!
As for MJ, though, the defining thing about her isn't that her hair is red, it's that she's a stunningly sexy, glamorous, fun-loving dream girl who falls in love with the nerdy hero despite being way out of his league, thus playing into the wish-fulfillment fantasies of the target audience. Arguably Kirsten Dunst's MJ wasn't that much like the comics character at all. She had the red hair, and she had the abusive father, and she was Peter's neighbor, but she didn't adopt a flighty, hedonistic party-girl persona to cover her pain, and she wasn't a model, and she didn't already know Peter was Spidey before he revealed it to her, etc. So sure, they got the hair color right, but that's not what defines Mary Jane Watson. If this version of MJ is truer to the comics' version in her personality, I don't think most fans will care what color her hair is.