I might be one of those that don't dress up and is slightly jealous of those that did. I usually commute, and wearing the costume already is not an option, while bringing the change of clothes is a bit of a hassle. Or maybe I'm just making excuses and I'm just lazy. Either way, I hope you have fun in your party.
Perhaps it depends on culture too. In Finland, people don't seem to mind dressing up in public (those who don't mind dressing up) so it is common to see people on the bus in a costume :)
That might be it. Asians tend to avoid standing out. Not all of course.