I went two years ago and most adults didn't wear them (Halloween night, too!) However, there were enough adults and whole families wearing them that you shouldn't feel out of place if you choose to.
My niece and nephew decided at the last minute it was too hot to wear theirs and they didn't feel out of place either (at 10 and 8). Keep an eye on the weather -- if it's shorts and t-shirt weather, a lot of store bought Halloween costumes won't breathe at all and the kid will want them off pretty fast. Might be easier to do a face painting or something instead (though my nephew had a bad reaction to the one he had at MGM -- niece had no reaction).