“Cartoon villain” here means “a villain who is just intrinsically evil and does evil things as a result.” Contrast this with real people, who generally have material or ideological motivational for the actions they take.

The left views NATO as evil not because it’s full of cartoon villains, but because it is an organization that consciously, due to material and ideological motivations, chooses to immiserate the global south for the benefit of its constituent countries’ ruling classes.