I think that the overall significance of Halberstam's idea about how the time period was a "mean time" is to reflect some of the dynamics that underscored the decade. On one hand, Halberstam makes it clear that the 1950s was a decade in which American prosperity and power was at its zenith. It emerged from World War II having defeated the Nazis, and was a nation that was "stronger, more powerful, and more affluent than ever before." Halberstam asserts that the flip side to this coin was the nation took this strength and channeled it towards the external world, a foreign element that was "pressing closer now than many Americans wanted it to." This particular reality is part of the reason Halberstam makes the argument that the "meanness" was a perceived show of strength.
This was in stark contrast to the vision of the nation prior to World War II. The New Deal was a time period of overwhelming comparative compassion, where government sought to help those less fortunate. However, it was conceived out of American weakness and frailty that was brought about by the Great Depression. Halberstam makes the point that the experience of America during the 1950s was one of strength and asserts that this took the form of "meanness" and "witch-hunts." The idea of being able to identify and target "the other" was a part of the cultural expression of strength in the 1950s. The significance of Halberstam's idea was to communicate how the strength and promise in the 1950s was so easily translated to targeting "the other" and silencing it.