Respectfully, I disagree. There's a line between cosplay and dressing up, and coming off as a creep. Although wearing certain clothing to a Children's Movie, would be a big no no, dressing up as a Character or something from the world, is a fun way to express yourself and can even add a little bit of magic for younger ones attending the screening. Although I wouldn't go full out cosplaying myself for the MLP Film, for the recent star trek & star wars films me and my mates were in full cosplay. If you're on your own, attending the movie by yourself, you might want to tone it down a little, but for a group of friends, it's always a hoot!
As much as I understand where your coming from and can see how it could appear "creepy' to parents when people go overboard, you are overreacting somewhat. Your use of the quote "Autistic Weirdos" isn't very much appreciated as well mate, and I highly doubt that it will have any, if all, effect on the Sucess of the film.