For a while in The Sixties, classic comic strip Dick Tracy tried to capitalize on the ongoing Space Race by sending its characters TO THE MOON! One character even got married to a "Moon Maid". After the moon landing, however, the moon, and most of the sci-fi elements, were dropped from the strip, and Dick Tracy went back to old-fashioned crimefighting...
... only for the strip to almost immediately stumble into a second Dork Age with its faux-blaxploitation/"street-wise" '70s style, including giving Tracy a god-awful mustache and longer hair.
Both of these periods (which may as well have been a single long one) carry the undercurrent of creator Chester Gould's increasing conservatism. Starting in the early 60's, Gould would regularly halt the story progression to rant about the Warren Court's decisions on the rights of the accused. Like the more obvious Dork Age mentioned above, these author filibusters would only stop with Gould's retirement in 1977.
The strip would eventually hit a third Dork Age from 2006 to 2011, which is widely considered to be its absolute worst period. A combination of artist Dick Locher's advancing years and the failure to replace writer Mike Kilian after his death led to five years that were marked by atrociously bad artwork and stories that tended to drag on for three or four months without much of anything happening, before everything was suddenly and unsatisfactory wrapped up in just a few days. Fortunately the strip was able to recover yet again when Locher left and was replaced by Joe Staton and Mike Curtis.
For Better or for Worse is widely considered to have ended on one of these, then restarted on another. It became a soapbox for the creator's very dated (and somewhat warped) views and beliefs.
Due to the rather static nature of Garfield, it can be difficult to tell whether the strip is in one or not. The general consensus is that the comic became horribly stagnant starting in the late 90's – during this period, nearly every single weekday strip consisted of Jon and Garfield at the table and most Sunday strips dealt with the spiders. The base broke when Jon and Liz became a couple (with many fans accusing the comic of jumping the shark), because it destroyed the potential for any future "Jon has a horrible love life and/or has insane wacky dates" jokes – a major theme of the strip.
More infamously, around this same time in the late 90's, the strip – which actually has quite a large ensemble cast – cut back on the appearances of nearly every side character (Nermal for instance), in favour of just Jon and Garfield with occasional Odie. Later, Liz's increased role led to even Odie only showing up even less occasionally.
With B.C., near the end of Johnny Hart's life he became very heavily Christian, and started shoehorning his fundamentalist beliefs into the comic, leading to some controversy (one infamous Sunday strip had a menorah morph into a cross). After Hart's death, his kids took over the strip and changed it back to its original light comedy format.
Funky Winkerbean was simple enough – a wacky comic that took place in a high-school. Years later, after a few in-universe timeskips, it became one of the most dark & depressing comics ever syndicated. Constant mention of cancer, death, characters trapped in a miserable world, and even a story arc of one of the major characters getting Cancer and slowly dying from it for the reading public to follow. Even its sister comic Crank Shaft (Which is in the same continuity) can't escape the constant gloom that all the characters suffer from. This isn't an dramatic exaggeration either; read Funky Winkerbean's trope page.
Doonesbury seemed to hit one in the late-Aughties as Garry Trudeau spent more and more of the strip's time focusing on the Iraq War, eschewing most of the political satire he was known for in favor of observations that were rarely funny. The nadir came when he spent several months on B.D. losing his leg and Alex's boyfriend returning to America after getting brain damage from an IED. Some Anvils Need to Be Dropped, certainly, but Trudeau was not nearly as maudlin with his strip during Vietnam.