Expressing sexuality? No. What they are expressing is 'hey, look at me for more than 5 seconds, I need attention desperately please.' That's all. No woman dresses like that and doesn't expect to get attention.
Anyone who is that reserved and embarrassed would shit themselves if the public saw them in lingerie. No, these girls are not at all 'reserved,' or they wouldn't be doing this.
They may not be reserved relative to others, I meant they are reserved enough to not dress like that every other day of the year. This one day being different in that it has created a social context in which dressing bizarrely is socially acceptable.
Anyone: "I'm not really a Vampire, I just choose to dress like one because I find them cool and am a fan of various vampire fiction works".
Everyone else: "Its Halloween, go for it and no one will glare at you".
You could say there is at least a tinge of exhibitionism to anyone who participates in the holiday's festivities, no matter their costume.
A funny guy can stick with what he knows and choose an amusing costume, as people would expect him too. Most people however tend to break pattern in some way and become some one or thing different that they could not be in a regular social context, and thus take the opportunity to "express themselves".
A male might dress as a 300 Spartan warrior because he thinks woman will dig the costume, or/and at the same time choose this costume because he digs the whole manly soldier imagine that such a costume is associated with.
A dude who dresses as some famous philosopher can display that he is interested in such intellectual endeavors, or a dude can choose a costume that exposes his muscles to show his accomplishment and dedication to exercise. Both costumes express values that normal clothes don't.
