Why it's a terrible idea ...
You're doing it for two reasons. First, you're legitimately trying to be witty and funny. The problem is that Halloween costumes are a terrible humor medium. Ninety-nine percent of "funny" costumes at best produce a forced courtesy laugh when you tell people what you're supposed to be. It's a joke that lasts about five seconds, but that you'll be wearing all night. And then will live on in however many Facebook albums you show up in. So you'll be explaining your joke costume, over and over again, for that same courtesy laugh, forever.
The second reason is that you just plain don't give a shit about dressing up, and you did the least amount of work possible just to say that you wore something to a costume party, just to get people off of your back. Because you're the kind of person who, above all, hates the idea of conforming to someone else's idea of fun.
Yes, you're incredibly witty. We'll all be over here if you need us.
To the rest of the partygoers who did show up in costume, that makes you a douchebag because it now looks like you just showed up for the free booze and/or food. The party meant so little to you that you weren't even willing to put ten minutes worth of effort into humoring the host or other guests. You might as well have showed up with an empty cooler an hour early, loaded up what you could eat and drink, and went back home with the loot.
It would have been much funnier and appreciated if you had taken the time to build a giant middle finger costume and told everyone to go fuck themselves the whole night. Which now that I think about it, I believe I now have my costume this year.
And be sure to check out Cracked's Page of Horror for more of John's musings on costumes.