Actually, now that I think of it, it's not the dressing up part that looks silly... It's dressing up and then doing poses like you're the character. Like when some fatty dressed as Sailormoon does the "oshiokyo!" pose. Or that one scary picture of that guy dressed as Sora doing that totally serious pose... freaky.

You can dress like a tard because that's fun, but there's only so much acting like a tard I can handle seeing before I cry.