I sailed on the Glory for Halloween a couple of years ago and had a wonderful time. We brought costumes and dressed up and would have felt very out of place if we didn't. They had a costume party near the bar in the casino and people had some very creative costumes on. I would say the majority of passengers dressed up, if not with a full-out costume, then a funny shirt, hat, etc. Everyone got in the Halloween party mood. Also, we brought decorations for our stateroom door and room. They should have a contest for best door decoration because some were outstanding, people were walking around the decks taking photos of the decorated doors. All in all, it was a fabulous night and I would suggest you bring an easy to pack costume, shirt.
P.S. I dressed up as Pippi Longstocking, ha!