Your yardstick of "good group behavior" could use some adjustment.
Derping just for the lulz should be saved for runs with friends. If you're in a pug group, it's better to have a little more courtesy and professionalism.
This. There is a certain amount of screwing around I will accept or even do myself in PuGs, trying to knock piles of mobs off ledges, set up some silly force pull + knockback type stuff. But then again I'm tanking, and even when I'm messing around, the net effect will speed up the run.
Derping around for the sake of derp or annoying other people however, is just impolite, as derp isn't usually a benefit to the group.
This sorc however, was clearly just not that good at the class. Probably thought they were doing a decent job, who knows. I accept these people, offer advice once, and if they don't take it to heart, ignore and move along.