Eliot Kid. Not a CN original show, but I associate it with CN because that's the channel I watched it on where I live. Horrid concept, every single episode would make you want to pull your hair out because they all have the same basic plot, Eliot and his 2 dumb friends imagines something that isn't actually real, makes everyone's lifes miserable by the end, the end. The characters are annoying, and the show in general is just painfully unfunny. The art style is decent, but the overall animation is nothing special. It came out at a time when I liked mostly everything that is animated, this was the first cartoon I actively despised, but I forced myself to get through it because Scooby Doo: Mystery Incorporated aired new episodes immediately afterwards.
But if we're strictly talking about CN originals.. it's hard to pick one because none of them are really all that bad, talking strictly about ones I actually watched more than 10 episodes of, but a lot of the 2010s cartoons are just so.. mediocre. We Bare Bears, We BABY Bears, Craig of the Creek, Summer Camp Island, all 4 of them are just so.. boring. Maybe if I were still a kid I would enjoy them more, but they feel like shows I would watch on Minimax (another cartoon channel where I live that airs a lot of pre-school content). Just boring cartoons with nothing interesting going on, with dull looking art styles, barely any background music, and none of the characters being all that funny or engaging to keep me invested.