In Victoria's Laboratory, the Negativitron created the first meanies out of cakes. These were fairly primitive enemies that would hop in the air.
There was also a larger, more humanoid meanie with a top hat and mustache who was a minor boss of the level Runaway Train. The Brainwave Machine was the last of them, being hijacked to create meanies before becoming sentient and being the main boss of Victoria's Laboratory.
In the Factory of a Better Tomorrow, Victoria's sackbots have been brainwashed by the Negativatron to be turned into Meanies. Sackboy somehow stops the hypnotism but they are terrified of Sackboy and flee from him. After going through a tub like contraption, they realise that Sackboy was saving them. There are spider like meanies that have a microphone on their backs and they fling fire. Probably the most prominent meanie is Copernicus the Guard Turkey.
Meanies invade Avalon Centrifuge's training center in Got the Hump but are repelled by the camel spit from the camel Sackboy is riding. They also appear in Flying in the Face of Danger and Huge Peril for Huge Spaceship, where one of them attack Huge Spaceship repeatedly.
Eve's Asylum is the first level to show the most commonly known type of meanie; a sackbot stickered red with the meanie's eye decoration and two meanie horns- curved. The Meanie infection also makes its first and only appearance here. The infection has drove various creatures and people insane. You go inside Dr. Herbert Higginbotham's body and eradicate the infection, only to find that Dr. Higginbotham is insane anyway.
This is where the meanies play their most important role. The only levels they don't appear are Set The Controls For The Heart Of The Negativitron and Into The Heart Of The Negativitron. All Meanies are presumably destroyed along with the Negativitron.