Of course!!! You paid for your ticket! I'd be upset if they didn't allow adults to wear costumes. LOL. But I think they do have some restrictions on costumes. Here is the info I found from the site:
Costume Information
All event attendees including adults may dress in their favorite Halloween costume and must follow these and other guidelines:
Costumes should be child-friendly and may not be obstructive, offensive or violent.
Guests may wear masks. However, masks must not obstruct vision and eyes must be visible at all times.
Guests who dress like Disney Characters may not pose for pictures or sign autographs for other Guests.
Costumes should not drag on the ground.
Costumes should not contain sharp objects, pointed objects or materials that may accidentally strike another Guest.
Costumes should not contain any weapons which resemble or could easily be mistaken for an actual weapon.
Guests of all ages may only wear costumes to the
Disneyland Resort Halloween event to which they have valid tickets.
Guests who do not adhere to these guidelines may be refused entry into the event unless his or her costume can be modified to meet the above standards.
Referenced:
http://disneyland.disney.go.com/special-offers/mickeys-halloween-party/