He does better for a couple of primary reasons. The first is that he's going to get a honeymoon and that's going to get him the benefit of the doubt in a way that Carter never did in 1979. People might even give him a break for having to deal with Carter's mess. The second is better relations with Congress. Mondale was a Senator and a reasonably well regarded one. Jawboning and schmoozing Congressional leaders was not one of Carter's strengths, but Mondale was a pol and would do better at it. Mondale is going to have labor in his corner, something Carter never did. In 1978, that still mattered. Perhaps he makes a push for the Humphrey-Hawkins full employment bill; that act might forestall any primary challenge from Kennedy. You have to remember that while Carter was not from the liberal/labor wing of the party, Mondale was a Humphrey protege and very much part of that wing of the party.
None of this is to say that he would necessarily win in 1980. A lot of things went south in 1979/80 and it is quite possible the same happens to Mondale. But it is possible that he does sufficiently better to make 1980 a contest.