If TV show spinoffs count as sequels, then I say Planet Sheen.
It takes all the things that those who actually liked Jimmy Neutron were willing to forgive, exasperates them, and throws a random character in a scenario that any cartoon could have been easily placed in and the show would be exactly the same. Just unfunny and generic and Sheen becoming completely braindead instead of naive and obsessed fanboy-y makes the show worse.