This topic is very sensitive. All cosplayers should respect others as they want to be respected. This goes for the photographers and the casual con-goers too. But we don't live in a world of happy narwhals and meadows of flowers.
I'm both buyer and "pull stuff out of the closet and modify it" cosplayer.
It all just comes down to whatever people think. Sure those who actually shed blood and tears are respectable because well, they put that much effort into it. But this does not make the cosplayer who bought the costume any less of a fellow fan. It's a battle between whether the two types of cosplayers are equal or one is better than the other. Ideally, we would say they are equal but in all honest truth it's a whole lot more when they have made the cosplay themselves.
Point is, don't care what you think is bad, just keep it to yourself. I'm not perfect because I do have opinions but guess what, I keep my mouth shut. Many people just don't know how to keep their mouth closed. There is a limit to what is constructive criticism.
Personally, unless they ask for a constructive criticism, it's better to just not say anything. Unless, you say something along the lines of:
"Hi, I really like your (insert name of character) cosplay. Hey, maybe you can try to do this here and it'll look a lot more perfect than it already is!"
Please, it's 2013 and I'm 100% sure con going etiquette forums or anything has been up since long time ago. Actually, just respect itself should have been instilled. Of course, not everyone is like this. Many people in fact are great, respectful con goers, it's that 30% or so.
To the respectful con goers, keep up the great work. To the 30%, not all opinions have to come out of the mouth.