Trust me, there will will tons of adults dressed up! We go every year and we always dress up (five adults and one small child). Some people get really into it and wear very elaborate and creative costumes. We always see LOTS of great costumes on adults.
Don't be shy about dressing up. You will get a lot of attention and positive comments, especially if you are a "unit" (like Star Wars), and after all, isn't that what a Halloween party is for?!
Last year our group was the Scooby Doo gang. I can't tell you how many times we got extra handfuls of candy from CMs who liked our costumes.
Have fun and Go for it!!