Something I won't hold against Dunst is that when she was cast, she definitely was one of the best actresses at the time to cast in the role; she was the IT girl of the most loved teen movies at the time, and of course she would have served better as Gwen, but it's easy to see why she was chosen for MJ. I personally thought Tara Reid should have played opposite her as Gwen.
However, I don't see the need for MJ to be supermodel hot, she just needs to ooze sex appeal while being hot, supermodel looks aren't quite necessary.
Regarding Jennifer Lawrence, there's just something about her that I don't feel she can bring MJ. Maybe it's because I'm already used to her as Mystique.