I liken Zero's rise to popularity and plummet to the bottom of my Faves list to the rise of Boba Fett's popularity. Fett was cool when he was a moving piece of background that no one knew anything about and said little. He had a reputation of disintegrating bounties, which added to that coolness factor. Then he went out like a punk, dumped into the sarlacc by a blind Han. But, even still, Fett was cool.
Then authors like Karen Traviss got a hold of him. Now he's living well into the Legacy Era, he's super-good at everything and paved the way for a whole battalion of Fett wannabes and the conlang Mando'a. Everyone loves him and wants to be like him and I see fanon character after fanon character that's just yet another Fett wannabe with Force powers. His popularity made him suck.
Now, Zero... He was cool as he had a dark past, he was borderline psycho, and said little. Then he became playable which I guess is the equivalent of Traviss writing Fett. Zero's now super-good at everything and everyone wants to be like him and there are fanon Zero wannabes running around. Hell, Axl idolized him! Then Zero lasted all the way through to the Zero series era, which is roughly the equivalent of the Legacy Era. His popularity made him suck.
However, his manga 'fore the author took a drastic new turn, was funny.