In the last episode he tries being good for once, and he's rather good at it. But everyone soon realizes they need him to be a bad monkey or else the City of Townsville is a very boring place.
I liked that episode. It's not just that he tries to be good, it's that he finally unleashes his plan that he's been developing all these years(if I'm thinking of the right episode) and it greatly improves everyone's lives to the point of boredom.
Also: There was once a Mojo Jojo greeting card that had just a huge block of text on the front to the effect of "I have heard that it's your birthday and so I have gotten you this card that is to be used to celebrate the anniversary of the day on which you were born!" and on and on. It was possibly the greatest greeting card ever.