Your query is meant to introduce the book and make clear why it's compelling. That doesn't require you to explain its plot in detail, particularly if the plot isn't the compelling aspect of it. On the other hand, you can't just write "My novel is called [TITLE HERE], it's very funny, you should totally check it out." So you need a core aspect of your novel - a plot thread; a character; setting; a catalysing event - which you'll be able to build your query around.
For example, The Hitchhiker's Guide to the Galaxy is certainly a humorous novel with a fast-paced, erratic plot which isn't really the main point of the book. What unifying element can we, nonetheless, find and focus on in a query?
Well, even though the plot zigzags like crazy, the central experience is that of Arthur Dent, milquetoast extraordinaire, being tossed and batted about the universe in a sequence of zany situations. So you could write about Dent, and what sets this off, and what he goes through:
All Arthur Dent wants is to be left alone, for his hangover to pass, and for those construction workers outside to please not tear his house down. The good news is, they're not going to. The bad news is, that's only because the entirety of planet Earth is blasted to smithereens. So Arthur Dent, sole survivor of planetary annhiliation, last human in existence, is feeling rather put-upon.
There - that's the premise. It explains the set-up for the novel; it introduces our main character; it explains what he wants - to be left alone and for things to be normal - and it's pretty clear that he's not about to get it. Now that we've got a clear premise and character, we can build on that to give a sense of what the rest of the book is like:
Arthur Dent discovers that, in absence of Earth, there's a whole universe out there. Much of it is loud, unpleasant, and trying to hijack him off on ridiculous escapades. Ford Prefect, whom on Earth he had believed to be a largely reasonable human being, turns out to be an alien marooned on Earth for decades, originally assigned there to write the planet up for the notorious intergalactic travel guide, The Hitchhiker's Guide to the Galaxy (his essay in entirety: "Mostly harmless"). Ford inducts Arthur into the ranks of the galaxy's interstellar hitchhikers - leading him from one mess into another, and always ever-so-slightly ahead of the jaws of disaster. Well, almost always.
There. That's a halfway-decent draft. You've said what you needed to - it's a funny book; it's about a boring guy who suddenly finds himself going through weird adventures in space; the tone is off-kilter and zany; the premise is a human becoming a "galactic hitchhiker" after Earth is destroyed. Premise; character; tone - you've got them all across, and you don't need to start explaining anything about probability drives, or start leading up to "The Answer to Life, the Universe, and Everything" in just a paragraph.
Two important notes:
I've resisted the (compelling...) urge to put in a list about the zany things Arthur's going to encounter. No "Arthur meets a rapidly descending sperm whale, the Creators of Earth, and a sorely depressed robot, and ultimately discovers The Answer to Life, the Universe and Everything." Why have I done that? Because that list is meaningless; if you haven't read it, it's just a list of random things. They may be interesting, they may be unusual, they may be well-written. But reading a list of them doesn't tell me any of that. So avoid the "teaser list" if at all possible.
Notice I've deliberately tried to write this mock-query with a tone and humor similar to that used within the book. (I'm not Douglas Adams and I'm writing rather quickly, so don't dock me for humor, OK? :P) This isn't always appropriate (and writing "in-character," as though a character from the book is speaking, is generally frowned upon) , but for a book that relies on tone and humor, this can be extremely helpful. It gets across why the book is worthwhile - so that's good.
Lastly, if you cannot find any core element of your story to focus on, you may have a problem. It's one thing for plot not to be the focus; it's quite another for there to be no plot, or incoherent plot, or a whole lot of "short" plots one after the other with no connection. Typical elements that bring the story together are often an initial set-up (like Earth blowing up...), a character (what does he want? how does he try to get it? what stands in his way?), a goal (if the final goal is clear from the start). If you can't find anything, your story itself may feel random, slapdash, incoherent. Your best recourse is to ask for feedback from people whose opinion you trust and respect. Ask them what they see as the core element; if you feel it's not strong enough, rewrite to put focus on it. Sometimes problems in a query letter do indicate problems in the work being queried about.
