u/400-Rabbits had a great answer on the "flashes of good humor" we can see "in the Spanish accounts of Motecuhzoma." Interestingly, the source u/400-Rabbits uses isn't very obscure. I would expect a historian of the Spanish empire to be familiar with it, especially if they're going to try and make sweeping claims about both Aztec and Inca society. I'm not familiar with similar observations of Atahualpa, though I'm definitely now interested in seeing whether they come out in a close reading of the Andean chronicles.

That said, understanding foreign humor is incredibly difficult. Making Peruvians laugh in Spanish is something I consider a serious achievement (and something I've only really done a few times) and that's in the present - when many of my Peruvian friends have consumed a great deal of American media. This difficulty only increases when you're talking about societies that don't share that sort of connection with your own (such as those of Pre-Columbian Mexico or Peru). See a brief interview with Dr. Stephen Houston, a Mayanist, on that subject.

All that is to say that even if it doesn't come through in Spanish historic documents (and according to u/400-Rabbits, it does in the Aztec case), it doesn't mean we can definitively say that these people had no sense of humor. That's an extremely radical claim to make on the sparse evidence that's survived to the present. Further, any historian or anthropologist that divides culture into "savagery" and "high culture" is, to put it mildly, way behind the times. I'd also be interested in the sources he provides for his assertion that the Spanish were "always laughing". The impression of conquistadors that comes across in the chronicles is not a very light a jovial one.