In truth the actual costumes created by most of the cast are pretty damned good, and I was familiar with the work of several of the cast members before the show went on the air just off of their reputation. The difference between a "serious" con and a not serious con has everything to do with the judging and rules of the con, and not the contestants. You'll see the same quality costumes enter both, but the criteria for winning is what makes the difference.
Think of it this way - there's a big difference between three random dudes on a couch deciding what their favorite movie is versus three notable film critics doing the same. The approval of the former is nowhere near equivalent to the latter. The same is true if you compare Wizard World Chicago and Anime Central -- both attact almost the same crowd, but winning a Master level award at Anime Central is much more prestigious than an award at Wizard World Chicago.
Same cosplayers, different judging processes.
The show has gone to serious events -- the Ottawa Pop Expo a few episodes back was one of them (even if they didn't show the prejudging for some reason). These events have the same variety of cosplayers enter them, but winning them is harder. But the inclusion of Wizard World shows is a joke if you want to talk about "serious cosplay" -- because they have no prejudging at all. Even the best cosplay judge can't get an accurate understanding of craftsmanship from someone being a half stage length away for less than a minute.
I criticize the show a lot, but rarely the actual cosplay. Most of them are pretty damned good.
(I can still give you examples of cosplayers at these cons, but I don't think you'd see a difference between them and what you see on other non-cast members on the show. Again, I have no problem with the most of the actual cosplays worn by cast members of the show)