Marceline is proving she's quite evil, no doubt giving that tantalizing view of her breasts and the promise of more lovely nakedness to some poor soul (likely either Finn or Princess Bubblegum). The teasing and her razzing expression are perfectly in-character for the vampire queen, and it looks like she' really enjoying it. Of course we're getting a better view of her complete form, so we're enjoying it quite a bit too. Thanks.