Now, even a kid knew this was silly. I mean, if we do something wrong, that makes us bad, but it doesn't automatically make us as bad as Gargamel. The line came off as an attempt to infuse moral sophistication into shows that portrayed all morality as totally black-and-white for the other 21 minutes, and thereby rang false. But even if it didn't ring false, it sure sounded that way after it was used again and again and again and again....
I think the death of this particular phrase came in an episode of "Batman: The Animated Series" in 1995, wherein Batgirl teams up with Catwoman. At the end, Catwoman is about to let the villain fall into a vat of boiling oil:
BATGIRL: Stop! If you let him fall, you're no better than he is.
CATWOMAN: Oh, grow up.
(She lets him fall.)
Sure, the villain didn't actually die -- nobody died onscreen, by order of the Fox censors -- but Catwoman's line was true catharsis for all of us who had heard that line 97.5 times growing up.
Rumor at the time was that the original script had the "no better than he is" line played straight, and somebody added the line "Oh, grow up" to make fun of the cliché. Don't know if that's true.
"Duckman" also had a riff on this trope:
CORNFED: In a way, we were no better than Duckman. Actually, I'm speaking metaphorically; we were a lot better than Duckman.
"Tiny Toon Adventures" also slammed Sat-a.m. cartoon clichés on occasion:
BUSTER: We have to help the flea family. But what can we do?
(A sign appears: "Cartoon Cliché # 1: Shrink Yourselves.")
BABS: It's so crazy it just might work!
(Sign: "That's Cartoon Cliché # 2.")