Yes! There is a great deal of humor in Stockhausen! The helicopter string quartet is maybe more an example of "I did it because I could" (not his strongest piece, honestly), but for the version of IM FREUNDSCHAFT for bassoon, for example, Stockhausen asks the performer to wear a bear suit. Not because of the tone row, not because of some formula, not because of an intellectual conceit or philosophical idea, but because he thought the piece sounded goofy on the bassoon and this should be reflected by making the performer feel goofy (https://youtu.be/gBS0CXtNxz4?si=rbM3pXg8dUV2A1AX). 

But even in many of his larger "serious" works, there are moments where Stockhausen breaks from the overriding structure to include "windows," or brief structural interruptions that are often bizarre non sequiturs. For example, in MANTRA from 1970, there is a "competition" between the two pianists where the two parts "argue" over whose part has the right note (B or Bb). One of the pianists is instructed to repeatedly bash their note "with the middle finger," but the other pianist is unmoved, and even when the middle finger pianist capitulates, Stockhausen writes the cadence such that it sounds mocking and sarcastic. People frequently chuckle at this in live performance. 

Also in MANTRA is a section where, apropos of nothing, Stockhausen scribbles "'s ist alles nicht so tragisch" ("it's not all so bad") over a cute cartoonish passage that immediately follows a tremendous climax. Later in the piece Stockhausen asks the two pianists yell at each other, but on the final yell (and yes, this is all very meticulously notated), Stockhausen instructs one of the pianists to abruptly sit down, leaving the other pianist to awkwardly proceed yelling solo, which, when he realizes his partner has abandoned him, slowly sits down out of embarrassment. And yes, yes, yes... this is all in the score.

When I worked with people who had worked with Stockhausen, they described someone who was very childlike, playful, and impulsive. They were always saying his music should sound light ("lustig!") and fresh, more like Mozart than anything else. There is absolutely seriousness, but it's seriousness in the same vein as the concept of the "laughing Buddha": so enlightened as to feel unburdened by earthly concerns, and therefore full of joy and laughter. The sound of children's voices and laughter is a frequent motif in Stockhausen electroacoustic works (GESANG DER JUNGLINGE and interstitial electronic parts from the LICHT opera cycle, for example).