I feel like there's a difference between humor and "silly." One is the manipulation of expectations to achieve comedic effect. The other is to manipulate the tone of the story and suspension of disbelief for comedic effect. RPGs can have humor and still be serious/mature. But if they fuck with tone and suspension of disbelief in the story, then the "butt of the joke" is the plot. Which makes it silly. Does that make sense?
Remember the human (This is a forum for a video game):