Penguins are terrific for doodling and cartoon drawings. For starters, they are fairly easy to draw; the basic shape is a long oval, with two arcs for wings, little paddles for feet, the beak and the eyes. Not a terribly challenging subject.
Penguins are also interesting because of the tuxedo look, and that is the other big reason they show up in cartoons and are fun in doodles or personal drawings. There's a nice feeling of debonair gookiness to penguins. They are clown-like in groups, and that paired with how well dressed they are, how could anyone not want to be able to doodle penguins, just for the sheer amusement?