Something cartoonish is exaggerated and overly simplified, like your cartoonish drawing of a cat or a cartoonish movie villain.
This adjective can mean "like a cartoon," as in the simple, unrealistic illustrations in a kid's book. It also describes things that are over-the-top and a bit silly: "Their cartoonish makeup looked fine onstage, but it's too much up close!" If the bad guy in a film grins evilly and twirls his mustache, most viewers will find him a little cartoonish. The word comes from cartoon, originally "a drawing on strong paper," from a root meaning "heavy paper."