It's not uncommon for political cartoons to have a simple message with each side, country, ideology or issue explicitly labeled. However, this does not necessarily mean the cartoon is of low quality: in most cases it's done to avoid ambiguity or at least the kind of miscomprehension that leads to angry letters to the editor.
Bill Watterson admitted that the Green Aesop in a Calvin and Hobbes story where Calvin and Hobbes took a trip to Mars was "pretty heavy-handed."
An in-universe example is Calvin's story "The Dad Who Lived to Regret Being Mean to His Kid", which he asks his dad to read to him.
Sometimes Mutts doesn't have a joke. Instead there's an ad for some save-the-animals cause, one that doesn't even feature the regular cast of the strip. It's like if the last five minutes of Seinfeld were replaced with a PETA infomercial.
Hey, Nemi-readers? Being cruel to animals is bad, okay? Got that? Too bad, because we're gonna repeat it a hundred times anyway.
Mallard Fillmore wants you to know that Liberals are bad, bad, stupid, stupid, bad, bad people. And they're stupid and bad, MMKAY?
And Ted Rall wants you to know the same thing, except about Conservatives.
Spoofed in one Bloom County arc where Milo has a nightmare about being a cartoonist with a black-hooded Torture Technician as his "boss", punishing him for typos and the like. After he wakes up, he starts to deliver a speech about how great is is that we have cartoonists, only for Opus to walk in with a level glare on his face and say "Oh, just stop."
On multiple occasions, Bloom Country would let the anvils drop. One strip in particular was a parody of Star Trek, and basically ranted about the shrinking amount of space available to newspaper comics.
Doonesbury is so heavy with some of its political commentary that some newspapers put it on the editorial page.