Let me file that under, "doesn't freakin' matter". In X-Men, she has an African accent in most of her scenes, and an American accent in others. According to the director, she simply forgot to do it in pickups.
In X-Men 2, she intentionally decided not to do the accent at all.
That ain;t a handicap, it's laxiness. She never wanted to do those films, therefore she didn;t care about her performance. Period.
Funny, in a couple of interviews, I saw her trying to talk up a storm spinoff movie.
Ironically, it looks like her idea went the other way, with a probably wolverine spin off.
Regarding the accent, the director was there, he should have simply either dropped it, or had her do it the whole movie through. He is paid to direct, Singer isn't exactly new, and Berry isn't known for being a pain.
She has already stated enthusiasm for doing another x-men movie.