As for She-Ra, I'm not sure about Jennifer Lawrence. I've only seen her in X-Men First Class and wasn't as impressed with her as many seem to be (I do intend to see Hunger Games once it's out on video, but haven't managed it yet).
Being almost wholly unfamiliar with He-Man, I can't comment on whether she'd be appropriate for that role. However, Jennifer Lawrence was nominated for an Academy Award for Best Actress in a Leading Role for her performance in the 2010 film Winter's Bone, which is out on DVD and available for instant streaming via Netflix; I would suggest that that's also worth checking out, especially in judging Lawrence's acting ability.
I've only seen Emma Stone in The Amazing Spider-Man, but she was pretty solid there, so yeah, I could see her as Teela.
I'd recommend checking her out in Easy A; it's a fun little film, and Stone's performance is delightful. She was nominated for a Golden Globe for Best Actress in a Motion Picture Musical or Comedy for that one. She's also quite good in Zombieland (which is itself a great deal sweeter a movie than its title would suggest). I'm told that she's also excellent in the films Crazy Stupid Love and The Help.