John Sides notes that, whatever else one can say about Mitt Romney's "even Jimmy Carter" comment, as it happens Carter isn't particularly unpopular: the last poll John found, from January 2011, had a 53/37 approval/disapproval split on Carter's presidency.
The point John is making -- that comparing Barack Obama to Carter isn't apt to be a very successful campaign line -- is most likely correct, so I'm not going to get into that.
But I will note that 53/37 isn't especially good for a former president; a similar 52/42 was 5th out of the 9 most recent presidents who Gallup polled about in October 2010. In February of this year, Gallup asked about it differently: were presidents above or below average? 38% said Carter was below average or poor, beating only George W. Bush and Richard Nixon of the eight most recent presidents (adding BHO and omitting JFK and LBJ from the 2010 survey).
Moreover, and getting back a bit to John's point, it seems at least somewhat likely that people might like Jimmy Carter more than they like his presidency and the policies he pursued, and that some of his presidency approval ratings are really a reflection on his post-presidency. I suspect the way to get at that would be to ask more specific questions: approval on foreign policy, domestic policy, economic policy. My guess is that Carter would be hurt by that (and Nixon, for whatever it's worth, helped).
The other point I'd make about this, which is not at all relevant to the 2012 campaign, is that my guess is that Carter's popularity won't outlast him. Carter is one of four postwar presidents who benefited from a sustained organized effort to improve his reputation after he left office. Two of those, Carter and Nixon, were essentially one-man operations; the other two were campaigns on behalf of Kennedy and Reagan. I wish I had more numbers easily accessible on this, but for what it's worth...Nixon died in 1994. In 1999, Gallup had a 22/41 above/below average ratio for him. Awful, but it's getting worse; this year, the ratio was 14/55. Now, it's not a huge change, and it's certainly possible it's just statistical noise, but my guess is that Nixon probably helped himself by pumping out seemingly serious policy books and making seemingly serious policy pronouncements during the 20 years he lived after the White House, mainly by making it likely that the most recent mention people had seen about him was positive. Once that ended, the odds are very strong that the last mention people have seen of Nixon is Watergate. Similarly, if the last thing you've seen about Jimmy Carter is the post-presidency do-gooder stuff, you're apt to think well of him, but if the last thing you've seen is about gas lines, inflation, hostages in Iran, and the 1980 election then you probably won't be as thrilled.