I think the statements is more justified with President Carter than with Ford. Jimmy Carter was an exceptional diplomat and negotiator, with the Camp David Accords and nuclear disarmament as real and lasting achievements during his administration. Most Presidents get blamed for the economy during their term, and it usually isn't their fault, but Carter was a little timid on dealing with inflation and job creation. He was a micromanager, handling things that the President shouldn't handle, like assigning parking spots at the White House, and some say he waited far too long to act in the Iranian Hostage Crisis.
Ford on the other hand never ran for President. He was handed a shattered Oval Office by disgraced outgoing President Richard Nixon. As a member of the same party as Nixon, it was difficult for him to fix the damage. He pardoned Nixon, perhaps for a valid reason - to protect the Office of President and not Nixon himself - but it sure looked bad to the public who wanted him held accountable. As if that wasn't enough, Vietnam, Laos and Cambodia fell in one month in 1975, and our long national tragedy in Vietnam appeared pointless, and Ford impotent to do anything about it. In this case, I feel he was more a victim of circumstance than lack of talent.
Both presidents are perceived as being ineffective because of their inability to address the economic and social crises of the time period. They both were perceived as smaller than the mandate of the time period. As disenchantment and malaise seemed to be on the rise, neither president seemed to be equipped to meet it head on and challenge it. Rather, their ineffectual natures seemed to add to it. Economically, both Presidents could not be blamed on causing the economic crises of the time, but they lacked the initiatives and demeanor needed to overcome it. Whether or not this is valid or invalid might be secondary to the fact that Presidents always receive a lion's share of the blame when things go wrong and credit when things go right. Certainly, the former applies to Presidents Carter and Ford.
Carter was considered to be ineffective for a couple of reasons. I think the main one is that the Iran hostage crisis happened during his presidency. The other is that the economy was bad. When bad stuff happens during a person's presidency, they tend to get blamed for those bad things whether they could have done anything or not.
Ford is mainly considered weak because he was not a presidential type and because he came in in such a terrible atmosphere. He had never run for national office and was just not that kind of a personality. In addition, he came in with only two years left in the term and with Democrats who were extremely angry about Watergate and anyone connected to Nixon.
I think that both men were largely victims of circumstance rather than bad leadrs, but you can read books arguing either way.
President Carter took the presidency at a difficult time. He was a spiritual man who had a calm demeanor. He was a southern born and raised man and was often made fodder because of his brother Billy's drinking binges and public demonstrations of stupidity that did little to provide support for Carter's intelligence.
The problems came quickly for Carter when the Iranian's took hostages in the American Embassy and it went down hill from there. America was having a fuel shortage crisis and people blamed Carter for the effects and felt that he was not strong enough to deal with the situations. The Soviet invasion of Afghanistan and his responses were also criticized.
Ford was never intended to become a president but it just had to happen. He was thrown into a position of decision making that he was ill prepared to take on. He walked into a political mess of Watergate and the Vietnam War. Many blamed him on the fall of South Vietnam and were angered that he had pardoned Nixon. They also saw him as a weak president.
Neither president were recognized for their strengths until after their terms ended. For Ford his presidency had placed him in a difficult position and his pardon of Nixon allowed the nation to begin to heal. Carter was also a man who had done his best but was later stated as having been too good a man to be a president.