That might make him (kennedy) poorly accomplished, but not the worst-
1. He basically stole the 1960 election, and was the product of a corrupt political machine (which was one reason he had no accomplishments to speak of -- because Congress knew this and basically decided to sit on their hands while he was in office).
2. His administration marked the start of an incredibly arrogant and dangerous change in U.S. foreign policy -- in which foreign heads of state were targeted for assassination by the U.S. government for any number of reasons (which was ironic, when you consider that LBJ went to his grave convinced that JFK's assassination was arranged out by Castro as retribution for the failed attempt on Castro's life).
3. The start of the U.S. involvement in Vietnam, which would eventually be one of the most destructive events (in political and economic terms) in U.S. history.