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?
OK…? As a general tip: when making boilerplate text to paste en mass make something like !!!__PLACEHOLDER__!!! to easily see if you need to change something while pasting ;).
That aside and going more into the meat of your post, I don't like stuff made with a purpose of being humorous. I do like systems that are open to make humour as an emergent possibility. For example: Orkhammer. Warhammer 40k can be surprisingly serious and in-depth, despite its superficial '80s hair metal awesome outlook, telling about people at their lowest. But concept of Orkhammer simply takes it into the lighter tone. It starts with 'what if Orks could infiltrate Empire of Mankind and despite their cerebral deficiencies steer everything from the shadows?' and you end up with something so crazy that you can't help but laugh all the way from creating characters up to playing as this motley crew.
Paranoia is also really cool. It takes post-apocalyptic sheltered society under the rule of not-really-malevolent-but-mainly-because-it-does-not-get-evil AI and encourages humorous play and outcomes. It can be grim. It can be outright cerebral with a good GM. But in the end, it should be like a Catch 22 turned up to 11.
Games that are made solely with the purpose of making you laugh often come as that guy who hears a joke, smacks you over the shoulder, and yells the punchline into your ear after about an hour or two. That's hard to get in a natural manner and keep the fire burning.
Added later:
As far as inspiration goes, I don't have any. I am pretty convinced that it is if not impossible than at least highly improbable to design it with such intent and succeed. To give an example of the funniest "we all got headaches from laughing so hard" I had with my group came from a Shadowrun game "Renraku Arcology".
It's a grim and very eerie place, an arcology controlled by AI that can withstand nuclear war etc… that suddenly sealed itself. No communications, no signals from the inside or coming through from the outside aside of a few skilled hackers who made it mainly to setup an IRC server inside for bragging rights. Visuals in the book are creepy as hell. We were a squad of corporate military forces set there for reconnaissance after some engineers were trying to breach one access point for us to get inside. It sealed itself immediately after we went inside. No contact from the outside. Nothing.
We realised that we are in a clean corridor, but there was nothing that was supposed to indicate that there was any life inside. As in "this place was crawling with over 200k people and we can't detect even bacteria on the walls" clean. Lights are started to flicker, GM informed us that we hear something like a generator fluttering at the other end of the section.
All this tension, genuine fear and suspense was concluded with one of the players trying to play 'the funny guy' movie trope and said
Rest of the campaign, despite maintaining a lot of eerie tones and creep factor was suddenly a big joke. We were cautious, but the moment we lost it completely was when my friend tried to play as a bait/decoy so that we could go around. He went into the open and yelled:
when exclaimed at something that looks like The Thing spliced with Event Horizon monstrosities was too much for us. Rest of the campaign was impossible to recover in terms of tone. We went all out as public officials here to deliver elec. bills to core AI.