It's best to just embrace the benefits of Halloween and not worry too much about how it does or doesn't make sense. Lucy actually knows the answer to why they wear costumes though: to hide from the demons, ghosts and other monsters that are rumored to come out on Halloween night. In that case, whether the costume-wearer meant it or not, the costume is doing its job. To be fair, a lot of modern costumes probably don't make much sense in this context either.

It's quiet this time of day. Lucy does have some music on, but the volume isn't too loud. It's true, Castiel usually does pop in when he's an angel, but she's not entirely certain that he's human until he makes his observation. A bright smile curves her lips at both the realization and the observation. "Every year and sometimes even when it's not Halloween," she admits then turns in a slow circle so he can get a better look at her costume. "What do you think? Are they going to kick me out of the ocean?" It's okay; she'll guide him into a compliment.