Went August, 2017 and August, 2018 both first parties of the season. Lots of people all ages dressed up. I'm in an ecv and my ecv was covered with purple and orange battery twinkle lights and Halloween garland. Adult daughter was evil queen and granddaughter was White Rabbit. It was hot and sooo much fun. You will see everything from the shirts to fully, beautiful costumes.
So, yes adults to dress up, but some dont. Decide for yourself!! Either way enjoy the party. (Look online at Disneybounding, to give you some ideas or Pinterest)