I wonder if Mad Men is more of a caricature. My family would go on picnics in the 60s and we'd clean up after ourselves. Maybe it was because my father was an Air Force officer and the USAF took a dim view of disrespect of other peoples' property. I remember AF bases as always being clean and well taken care of.

I'm not quite as old as you, but I'm old enough to remember when not littering was considered laudable and not simply common decency.