No you won't look daft - the more outrageous the better. They have a fancy dress competition as part of the show. When I went, there were the usual nuns, nazis and Marias, but one or two had gone along as things like a bee sting or a mountain. Dresses made from curtains, oversized guitars - anything goes, really!
If you go in normal clothes or as a nazi, take a torch. Then when they're looking for the family in the convent graveyard, you get to shine your torch around the auditorium and look for them. Otherwise you have to crouch down and hide behind the tombstones (aka the seats in front).
Enjoy!