She is tone deaf. When she was caught singing the Star Spangled Banner on an open mike it was clear that she is completely tone deaf. So, it isn't surprising that she doesn't do accents well.
But . . . when they were trying to rehabilitate Bubba back around the time of the whole Monica thing, didn't they sing in the choir at that church they went to? Or was that just Bubba? Or did they tell her just to move her lips?