Are you suggesting if they make a movie about black characters, you can't think its overrated without being a racist?
No. I'm saying the sentiment that the movie is overrated because of black characters is racist.
If you want to say it's overrated, that's fine. If you want to give a reason, that's also fine. If the reason is the skin color of the actors, that's textbook definition of racism.
Most people seem to acknowledge that it was a good superhero movie. Or at least as good as most Marvel movies. Probably even one of the better ones. That is a fair appraisal, I think.
The suggestion is that progressive-minded critics are giving it excessive acclaim because they want the first big marvel superhero blockbuster starring a black hero, with a majority black cast, black director, etc. to be very well regarded. Particularly when awards programs have gotten negative attention in recent years for not being diverse enough.
It seems to me that the nitpicking isn't that the movie is bad and only getting praised because the cast is black, but that critics hesitate to knock it because they don't want to be called racist and that they would nominate it for an award to show how not racist they are. I don't think that's a racist suspicion. But I'd concede that racists would use any avenue to tear down a successful movie starring minorities that they can.
This is just a variant of the timeless "sjws are the real racists" argument that has never made any sense. ---