Disneyland opened in 1955. The term "Happiest place on Earth" was not nearly so outlandish in 1955. Many nations were still cleaning up the rubble of World War Two. We were just ending the Korean War, and everyone on Earth woke up every morning knowing that this might be the day when some nut-case pushed a button somewhere and destroyed the world with nuclear weapons. I know that sounds melodramatic to you whippersnappers, and, yes, politicians used fear of the "Communist Juggernaut" to whip up fear in a less sophisticated population, but I was there, and to us it was real. In 1955 "Happiest place on Earth" simply implied a place where a family could spend a whole day together without worry or want.
I think we may need to hire Poppy Everett as our historical expert! Great perspective on the Happiest Place on Earth moniker. As someone who did not live through the fifties Cold War experience, my vision of the fifties is of an idilic time in America (I watched a lot of Happy Days as a kid, and we had American Graffiti on Video Disk - must have seen that one a hundred times).