Funny how you all name one & the same modern shows 95% of which I didn't see a single episode of, but lol... guess you know better.
Though few of the worst (or not very good) shows from some ago would be:
The Fruitties - originally a Spanish cartoon from the 80s, saw it a lot on Cartoon Network in the 90s, watched recently & it was awful. Plots are Ok, whole idea & characters as fruits is pretty interesting, voices (English) are great, intro song is cheesy & catchy, background music is nice, but the cheap animation just makes you forget all that. Watching it it feels like as if it was made by those who make anime but only much much cheaper - whole show is like composed out of still images (where only mouth is what moves all the time) & loops with frequent coloring errors. Also bad timing for many actions, like you can see a loop of character blandly & unrealistically walking for like 5-10 seconds, like if it's extended to be a filler.
Top Cat - same cheap animation like in all HB's 60s classic. It gets annoying to see characters in still poses nodding their heads all the time during talking.
Wacky Racers - same pattern plot for every episode & same unfunny gags over & over.
That Coyotee & RoadRunner shorts - awful unfunny cartoon of one character failing to catch the other one all the fawking time no matter what he does.
Rugrats / Hey Arnold / Angry Beavers / CatDog / etc - feels like they all were purposefully made ugly (and sometimes gross) to fit in to Nickelodeon standards... No wonder, I hated that channel for not having anything I would like.
PB&J Otter - self explanatory.
As for a modern shows, then, well, MLP:FIM is one of the worst around thanks to bronies/cloppers that overrate it, annoy with pony porn & give a bad name to already annoying (especially acrid) show.
Oh & did anyone hear/watch Peppa Pig? If you didn't then imagine I never mentioned it in the first place!