The shock of Kennedy's assassination was not due to his popularity, but because the US President had been assassinated there was a sense of shock and confusion in regards to how one of the most powerful countries would operate without a president. By saying this it also reinforces the fact that Kennedy was not universally popular, and there were intricate factions who opposed his presidency as well as the continuous supporters of his Presidency on the other hand.
Kennedy was supported by pockets of society throughout America who saw him as a charismatic young leader who had a vision for America. President Kennedy aimed to deliver a "New Frontier" which aimed to put an end to discrimination and improve general wellbeing for all Americans by introducing things such as funding for education, Medicare for the elderly, government intervention to halt recessions etc. It is evident that President Kennedy did have significant numbers of importance due to the fact that there are large numbers of crowds attending to his address before the 1960 election campaign, but this image alone does not account for the perception of a entire nation.