It's funny. I don't remember blatantly hating any cartoon characters as a kid. But I Don't like those characters that replaced Brock and Misty on Pokemon. When they replaced Misty I stopped watching the show. (As you might be able to tell I grew up in the 90s. Pokemon ruled my childhood...and beyond) Mostly I don't hate any of the characters I watched as a kid, just the new characters that replace them. Perch Perkins from Spongebob is another example, the Realistic Looking Fish Head was one of the best parts of that show, when I tuned in to some Nicktoons and saw they replaced him with some generic TV Reporter character...there was hell to pay. There were a lot of awful cartoons that came and went that had crappy characters...other than that I can't really think of any.

BTW Didn't they make Scrappy the villain in that live action Scooby-Doo movie a while back?