Stupid comments on HN are likely to be very different in form than stupid comments on Youtube.
They are more likely to be written by smart-ish people who are either acting belligerent or trolling.
Reddit might be a much better source of highbrow stupid.
Any site that refers lots of new traffic to HN might also be considered as a source of potentially stupid comments. A markovian discriminator could get very good at identifying, say, TechCrunch-style comments. ;-)
But with Reddit you have to triage the comments, because there are always some intelligent ones. I think 4chan might be the best choice. I've heard it described as "where smart people go to be stupid", and I think that's reasonably accurate. ICHC would be good too if you ran it through a LOLspeak -> English translator first.