Q: How can you tell if a joke is funny?
A: Someone laughs at it.
When a person says "That's not funny," all it really means is that that particular person did not laugh. It does not mean that the joke is not funny. And "funny" is not synonymous with "in good taste." Slapstick comedy, for example, is often about people getting hurt. And good people don't laugh at other people's misfortune. . . unless it's funny!
I had never seen or even heard of Jimmy Carr until I read this National Review piece by Charles C.W. Cooke, whom I consider to be a cogent thinker and a fine writer. Then I watched Carr's linked, two-minute video clip, which I found very funny indeed, even though it's about the Holocaust.
I don't mind if you don't find it funny. I hope you don't mind that I do.