The cartoon I've always recalled the least fondly from my childhood would be the Littlest Pet Shop spin-off cartoon from the mid-90s. As a kid, I was an avid LPS collector, so I routinely forced myself to watch it, but, try as I might, I could never force myself to like it. It struck me as the work of a production team who were keen to emulate the high-engery formula of the "Steven Spielberg presents" cartoons of the day (Tiny Toon Adventures, Animaniacs), but didn't understand it, and thought they could get the job done just by being as noisy and chaotic as possible. The resulting cartoon was an utter mess - annoying at the best of times, and horribly, horribly confusing at the worst.
Then there's The Kwicky Koala Show, notable mainly for being one of the last projects that Tex Avery worked upon (and a bit of a bum note to end his career on, quite frankly). Generally speaking, cartoons from the early 80s aren't renowned for their high concepts or production values, but a lot of them still manage to be entertaining in a "so bad it's good" kind of way. Kwicky, on the other hand, was both bad and mind-numbingly dull. The titular character was a Droopy-esque koala whose big gimmick was that he could evade predators (more specifically, a hopelessly neurotic wolf) by moving at lightning speed - thanks to the cut-price animation techniques however, what he'd actually do was disappear in frames of animation and then reappear elsewhere in the scene. And that was about it. There was also a supporting segment about a lynx who made the most appalling wisecracks and had a tendency to wiggle his eyebrows provocatively at the camera, but it was still infinitely more entertaining than anything Kwicky himself had to offer.
Finally, I'm compelled to give (dis)honorable mention to Disney's House of Mouse...it wasn't godawful exactly, but it was a terrible waste of a good idea.