Season 5 was indeed bad since the story was essentially over by season 4 with the deaths of you know who. It was pure fan service of stuff they didn't get to show all this time but they didn't contribute in anything.

Season 4 was also bad for the first half with the space quest stuff, since it had nothing to do with what we were fond of and was basically using time travel to undo the bad, and went as far as making the past turtles to simply leave Earth with no questions asked. It was super lazy.

Season 3 is where the big problems began, since half of it was again the turtles being in the wild and having pseudo horror adventures with nothing we were fond of, before returning to the city and bringing back the status quo.

So yeah, only the first 2 seasons were good, and even then most of the hilarious gag comedy was gone in season 2 and half of everyone had become mutants as the only way for the show to keep having something new happening.