Big difference.
I don't have a problem with people who dress on festive occassions like Halloween or going to a Convention.
This 'Zombie Squad' sounds like an organized tree house club for adults. They took it too far is why I think it's lame. Those other people you mentioned, the ones who dress up as Spider-Man, Batman/Joker or characters from Star Wars do so at conventions and or Halloween. Zombie Squad seems like they're trying to be 'leet' or some crap.
I don't know, just the way you said that Zombie Squad was there it just made it seem like they are some "big thing" which makes it even more embarassing for them as well as people/fanboys who get excited over them as if they're celebrities or something.