The lawn mower is tiny in order to comply with Plankton's height and is red and gray in color with maya blue wheels in the rubber tires. In "Shopping List," the lawn mower is red with a gray handlebar and wheels.
After SpongeBob and Sandy get some fur from the Yeti Krab, Plankton drops down onto the Yeti Krab and uses his lawn mower to shave some fur off. The noise from the lawn mower wakes up the Yeti Krab, who shakes Plankton off his head and proceeds to fold him into a paper hat to cover the bald spot the lawn mower made.
It is unknown why Plankton has a lawn mower in the first place, considering his residence is his own restaurant, which has never been shown to be surrounded by any kind of plant-life.
Although most of the people whom SpongeBob and Patrick accuse Mr. Krabs of stealing from having done nothing to provoke him, it would be natural for him to steal from Plankton, given all the latter's (failed) attempts to obtain the Krabby Patty secret formula.
Since SpongeBob knows that this object is a lawn mower, "Life of Crime" takes place after "The Curse of Bikini Bottom," in which he didn't know what a lawn mower was.