I'm American and I'm also somewhat dissappointed in the homogeneity of the accents in the X-men movieverse because one of the things I love about X-men is the diversity, but I don't think the characters were deliberately "Americanized" with the the idea being that "Oh well we have to make everyone American or else the movie won't be marketable enough, or won't make enough money, etc."
If that was the case then why did we get an English actor playing Magneto with this his own accent and an English actor playing Xavier with his own accent and now we're getting an Irish actor and a Scottish actor faking English accents to play the younger versions of these characters, when neither character is English in the comics? Xavier's even supposed to be be American, so he got "De-Americanized". Why even bother trying (trying badly but still trying) to give Storm some kind of "African" accent in the first movie or getting an actor who could do Nightcrawler's German accent in X2? Both of them could've easily been Americanized(from the beginning in Storm's case), but they weren't.
I think they just didn't care...that much about the accents. I think they just hired actors that they liked and if the actors could do the accents of the characters, then that was just a bonus to them. Maybe, I'm just speculating. I'm not saying that's good either, I wish there was more effort in choosing actors who are good and either can do the right accents or already have them.