A Quinnipiac University poll, taken May 23-30, 2006, asked 1,534 registered American voters to pick the best U.S. President of the last 61 years.
Jimmy Carter came joint fifth with Dwight Eisenhower, behind Ronald Reagan, Bill Clinton. John Kennedy and Harry Truman.
The same poll also asked for the worst president - George W Bush won with over one-third of the votes.
A Washington College poll about presidential greatness, taken 11 February 2005, asked 800 adults nationwide, "Thinking about all the presidents of the United States throughout history to the present, who would you say was America's greatest president?"
Jimmy Carter came eleventh in the poll.
A Gallup poll about presidential greatness, taken 7-10 February 2005, asked 1008 adults nationwide, "Who do you regard as the greatest United States president?"
Jimmy Carter was ninth.
An ABC News poll about presidential greatness, taken 16-20 February 2000, asked 1012 adults nationwide, "Who do you think was the greatest American president?"
Jimmy Carter was thirteenth.
So it seems that Americans certainly don't consider him anything like "the worst president ever".
However hard I look I can't find a poll of Spanish conservatives on the subject.