First time we went to MNSSHp in 2007 we dressed up (was about the 20th) and honestly we only saw about 12 other adults in fancy dress. I'd read various boards and thought the majority would be in fancy dress. Not being an over-confident female at the best of the times I changed back into my 'own' clothes after about two hours and dumped the costume in a trash can.
Last year we went on the 29 October and about 30% of adults were in costume with about 90% of kids. DH, DS and me all wore the MNSSHP t shirts we had bought and felt much more confident - DD still went as a princess obviously.
If you love to dress up and have masses of self-confident then go for it. If like me you like the idea but haven't got the bottle unless with the majority then it may not be for you.
If I was to do a costume this year it would be a very subtle one not an all out 'proper' costume.