She looks like a real world girl, not a pin up. They might pretty her up a bit more.
But I can buy her as a high school student. She looks a bit like an older Ginny Weasley...
MJ, however, is supposed to be gorgeous and way out of Peter's league. Emma Stone is much more of a MJ type than this girl is. She's cute and everything but she's not the super-model, drop-dead, gorgeous that MJ is supposed to be.
It's pretty terrible casting, IMHO.
They didn't have the money to hire a pin up after spending it all on the visual effects.