I love the outdoorsy look as she steps away and flashes back her lovely smile. The flannel shirt, warm hat, and big comfortable and muddy books show that she's rugged, no stranger to the outdoors, but her slender form and beauty tattle on the "normal" sweet and easygoing teenage girl. Adaptable. Best of both worlds. No wonder Dipper finds her so attractive. And this is the kind of candid shot that he could pin up on his wall or put on his desk unashamedly as he would for any friend.
Good thing Mabel's not the jealous type, but she's trying hard enough to forge some friends on her own (including of the opposite-sex kind, but her luck hasn't been that great).