Well, Kerouac was writing about the "beat" section of his generation, which was, despite it's fame, always a minority even in its time. It's not for nothing that it never became dominant. Most people - then or now - did/do not want to live like that, even if they get caught up in the idea of absolute freedom to do as you want when you want for the duration of the trip... it's a seductive idea, but when push comes to shove it's no way to live, and most people do recognise that. So I don't think it is a "snapshot of postwar youthful Boomer mentality", just that of a small section within it, even if their dream did make a lot of (other) people smile for a while at the dream of absolute freedom.

As you say, this was a post-war generation, and they were revelling in a new-found freedom, not just from war, but from the strict social conventions that had dominated pretty much all societies since forever. So, yeah, a section of them took it to extremes and had a whale of a time. For a time. And others looked in with a variety of reactions. But I would caution against over-generalisations.