Yeah, it's very individual where the line is for different people. For Brooks, his objection, as I understand it, was that, while he didn't feel uncomfortable mocking the Nazis themselves in his own work in the context of a comedy, Life is Beautiful directly depicted victims of the Holocaust and framed it (in his opinion) in a way that was overly sentimental and optimistic. As I said though, I've never seen the film, so this is just what I've heard secondhand about other responses to it and his view is obvious far from the consensus.