Everyone has judged someone as soon as they gave them the up and down (judging someone by their appearance) and has probably been way off from what that person is actually like; possibly you could have been correct but most likely you weren't.

Of course someone shouldn't be judged solely by how they present themselves; we can't just make these assumptions and go from them as if they actually had merit in their origin.

Point being - We cannot be a good judge of character within the time frame of five seconds when meeting someone for the first time. So don't be so quick to judge someone xD

Personally I am not trying to give off any impression when I meet someone, I don't dress to impress :p I dress comfortably and in what I feel comfortable wearing. But as for dressing in a manner suitable for social engineering (That is to speak of, persuasion) I'm quite sure that if one were to dress a certain way they could easily give off an impression not similar to theirselves in order to gain favor with a certain party, by being a poser in other words. I would not consider it poseur if one wanted to gain favor solely to influence a group rather than gain friendship and a sense of "fitting in", genius is what I'd call it.