I was going to ask readers when Peanuts jumped the shark, but a Web search reveals that blogger Jamie J. Weinman has already answered the question in "Peanuts' Peak"—and his or her feelings align with mine exactly:
In the end, I'd pinpoint the end of the peak period around 1975, with:Read the whole thing.
a) The introduction of Spike and other Snoopy relatives
b) A slight change in format, with thinner panels and thus less room for interesting visuals, and
c) The Snoopy and Peppermint Patty stories have begun to take up so much time that there's no more room for interesting Charlie Brown stories like the Mr. Sack series (from 1973).
Weinman also points to the classic "Against Snoopy," a scathing piece in New York Press (yes, that rag) by Weekly Standard editor Christopher Caldwell, who says the dog dragged the strip downhill:
Snoopy was never a full participant in the tangle of relationships that drove "Peanuts" in its Golden Age. He couldn’t be: he doesn’t talk (all his words appear in "thought bubbles"), and therefore he doesn’t interact. He’s there to be looked at. He appeals to readers through the many variations Schulz can play on Samuel Johnson’s quip: "Sir, a woman’s preaching is like a dog’s walking on his hinder legs. It is not done well; but you are surprised to find it done at all." Once Snoopy stood on his hinder legs, "Peanuts" got handed over to this lower order of humor. Thus we get Snoopy standing against a lamppost for three frames until Linus walks up in the fourth frame and says, "The worst thing a person can do is waste his life hanging around street corners." Imagine a dog playing tennis or golf! Imagine a dog reading a novel and saying, "There’s no way in the world that Anna Karenina and Count Vronsky could ever have been happy."Actually, I kind of like those blackout jokes. But there's no question Snoopy's cuteness was tiresome by the Seventies when, as Caldwell notes, Schulz would have him "dance through a whole Sunday strip before saying, 'Feelin’ groovy.'"