During the regular Halloween period I can't remember seeing many people, if any, wearing fancy dress. I think there were a few more kids with costumes on 31st October, but not large numbers. Of course, this may have changed as it's been three years since we visited DLP at that time of year.
If you are going to either of the Halloween Soirees (hard ticket events on 31 Oct and the nearest Saturday night to it) there will be loads and loads of people (adults and kids) dressing up. In fact, we felt out of place not dressing up!
Regards
Rob