Billie put in the bare minimum of effort into looking presentable for this party (a loose ponytail, blue sweater and jeans) because she is in a Mood (a nat [1]). The party is happening where she lives though and if it's really going to be the last chance for her to party before a zombie horde wrecks everything, then she's not passing it up even though she is not in a partying mood at all. She does try to at least not let her rain clouds get in the way of anyone's good time and hangs back in a corner smoking most of the time.

At some point, she leaves and changes her outfit entirely and is notably a little bit cheerier in her denim vest and giant sunglasses.