Ernie Bushmiller's Nancy didn't change much over the decades once he hit his stride; strips from the 1950s look exactly like ones from the 1970s. He didn't include topical references very often, but something about hippies struck a nerve, and he did a lot of gags about them in the late 60s. Note, however, that February, 1968 is kinda early for a mainstream newspaper kid's strip to mention the shaggy counterculture. Basically, Nancy will do anything to avoid bathing.
UPDATE: On a formalist note, please appreciate, again, the Mondrian-like perfection of Bushmiller's composition. What, for instance, is the window, a detail totally inconsequential to the gag, doing there? It's there for balance. Without it, the panel would be lopsided towards the lower left. Sublime.