J.F.K. was a mediocre President. For two and a half years his position on civil rights was legalistic—he stood up for enforcing court orders—until the dramatic images from Birmingham in May 1963 forced him to describe the issue as a moral one. The civil-rights bill he then introduced into Congress stood little chance of passing partly because Kennedy was unwilling to spend the huge amount of necessary political capital. For those who believe he was on his way out of Vietnam when he was assassinated, how to explain the dramatic coup three weeks before his death that overthrew the government of Ngo Dinh Diem and pulled the U.S. ever deeper into the quagmire? Kennedy’s main domestic accomplishment was a tax cut; his main foreign accomplishment was avoiding nuclear war over Soviet missiles in Cuba (his finest hour).
All I'd observe is that it's actually common for invocations of the heroes of the past to be someone vacuous. FDR-praising liberals typically don't have the details of the New Deal legislative program in mind or any particular inclination to reinstitute something like the National Industrial Recovery Act. Similarly, it's been noted ad nauseam that the historical Ronald Reagan doesn't greatly resemble the Reagan Myth the right has constructed. At the end of the day, there's probably not much point in worrying too much about the strict accuracy of our comparisons.