Here's a mouth drawn in the style of American cartoonist Milt Caniff. According to artist and comics historian Ron Harris, "Drawing the lips too round or too full can detract from the "man's man" look realistic artists usually strive for. Impressionists Noel Sickles and Milton Caniff circumvented this potential pitfall by drawing a thin upper lip in shadow, reduced almost to a line, while indicating the lower lip only by the shadow it casts on the chin."Ron's blog post "Mouthing Off" has a lot more examples of ways the great comic artists simplified the mouth for maximum clarity and expression.