I profoundly disagree. I have known Carter since before he was governor of our state (and a sanctimonious, micro-managing one he was, too. Walked around lowering the thermostats in the Capitol building, gave the GBA fits because of course they were balanced and he threw the entire HVAC system out of whack.) As a fellow Georgian, I always wanted to give him the benefit of the doubt. I too used to say that he was a good decent man but a bad president. But his conduct since leaving office (starting with the horrible interview he and Rosalyn gave back in 1980 where they trashed Reagan just out of pure spitefulness, though his much-publicized spat with the Southern Baptist Convention, his unsolicited attacks on Bush, etc. etc.) I can no longer say that in good conscience.
BTW, he isn't usually "called to act as mediator" - nine times out of ten, he "volunteers himself" for the job. As he did in North Korea, where he freelanced an agreement without the knowledge or approval of the U.S. It irked Clinton no end that he did it, but his inept negotiations made the NK problem much worse. When he's "called", it's usually by a socialist or Communist government seeking to embarass the U.S. (e.g. Venezuela). And the Nobel Prize, I fear, was simply the Committee's way of giving the U.S. one in the eye.
I think you are both absolutely right on the mark about Carter!