This is off topic but not really. In one of his books Stuart Maconie talks about filming in Yorkshire and someone (not part of the production) made a 'joke' about the Moors murders. He said the whole crew, consisting of arty, edgy London creative types fell into a horrified silence. Noone laughed.
I think this is what bothers me most. Jimmy Carr's audience laughed and applauded. Maybe some of them realised any ironic tone he meant, maybe some will think about it later and challenge themselves and find out more, but I am very sure that many of the audience and those who've seen it since will laugh because they don't give a damn about the genocide of the Roma and Sinti. There have been comments on this thread (thankfully only a few) that talked about negative experience of travellers/ some traveller behaviour in a way that is utterly grotesque in the context of the mass murder he referenced.
The hate is real and it doesn't need feeding in the name of 'humour'. I am certain, though, that everyone in that audience would have personal a cut off point where he went too far, because everyone has one. I applaud those standing up for the memory of hundreds of thousands of innocent people who have been turned into a cheap "Oooo this will end my career" joke by a dark haired version of Max Headroom. I'm sure he won't be offended by that comparison, nothing off limits, remember?