Having had to write this review - http://6d6rpg.com/rpg/reviewing-christmas-ninjas/ - I've been thinking about what does and doesn't make for a humour RPG.
To clarify, a humour RPG is a product written where the primary focus in on making the players laugh. Drama, high adventure, tactics and mathematical optimisation can still happen, but they take a back seat to giggles.
For me, a humour RPG is at its best when it's simplest. Have one core idea from which humour can be derived, maybe a couple of other minors points as support. The game needs to set things up and then get out of the way and let the players do the rest of the work. The players know what they find funny so the product need only present the starting point.
Christmas Ninjas does not to this. It's overstuffed with so many unformed silly ideas it can't work out what it itself find funny.
Reddit, I'm thinking of funny games I want to write. Can you give me examples and advice on RPG products that have inspired fun funny times?