I'm surprised that Kennedy has such high approvals. If he was president today I think most liberals would be appalled.
How so? Moving the highest marginal tax rate down from 90%? I don't think that there's anyone (outside of people like opebo) who would support return to Eisenhower-era tax rates.
His actions during the Cuban Missile Crisis? Hardly controversial by any standards.
Championing (though he died before it could pass) Civil Rights reform? Excellent measure, and hardly controversial on the left.
Sending "advisors" to Vietnam/assassinating Diem? Considering how few people knew about either (the war there didn't gain a significant American presence until '65) I can't see how it would "shock" many people. Besides, initially, there wasn't anything
wrong with holding to the Kennan Doctrine and supporting the South Vietnamese regime. It's true that Kennedy's final actions there, signing off on Diem's murder to put a more America-friendly puppet, was reprehensible, but Diem was a real piece of work, anyways, and the images of self-immolating Buddhist monks were seriously detrimental to American commitment in Vietnam.