I don't know if you know this, but she is half deaf, with no hearing in one ear.
I'm not sure why, but supposedly it has affected the way she talks sometimes.
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.