I think it goes back further than that—maybe right after WWII? But every time the boys in Peanuts travel somewhere, including going to camp, they wear a sailor hat. I believe the caps should be colored white too, not brown.
Your point is well taken, but in the case of Marcie she, Peppermint Patty, and Franklin go to a different school than the rest of the kids because they live on the other side of town. So Charlie Brown might not know as much about her as some of his other friends.
I took it as that was the joke—P.P. thinks the phrase is “vulgar” when it isn’t.