I do think that we can say both that Peanuts was a tremendously important, seminal and (more importantly) funny strip and that it had not been at its peak for the past decade or so. (Of course, to me, even second-rate Peanuts was miles about first-rate Cathy.)
I personally think the strip had more than one high point: the original strip was great (and, if you’ve seen the often-reprinted-recently first strip, you can tell that it was quite different from what Peanuts became later, both in tone and art style) and it hit another high point, in a slightly different mode, during the late '60s through the early '80s. To my mind, a strip that not only lasted nearly 50 years but had several distinct periods of greatness shows a gifted cartoonist who never stopped tinkering with the strip and was always open to new ideas and approaches.
I mean, compare Peanuts of last year with some of the other long-running strips (I’d choose B.C., Cathy and Family Circus, but YMMV) and see who comes off best. In the elephants’ graveyard that is the funnies page, Sparky Schultz kept doing good work for five decades while his competition bought jokes, farmed out the art, and went golfing. I think the difference shows.
…but when you get blue, and you’ve lost all your dreams, there’s nothing like a campfire and a can of beans!