I think humor is appropriate.
It doesn’t pass for humor in my view. It’s just a dumb, childish retort. Let’s look at an extreme example to illustrate why the retort is foolish. Suppose it was a pic of Hermann Goering holding a sign saying, “We eat Jews” and the response is to note that Goering is fat. It’s just a dumb take. The response ought to be about how abhorrent it is to call for eating people or decapittimg them.
Notice how because of his dumb tweet we are not discussing the important issue of being able to advocate for a position without the threat of violence. We are talking about Paul Graham’s dumb tweet.
When get into a fight with mud slingers you come out dirty.
It'd still be funny though :)
Tangentially, this is why Twitter has never made sense to me at a structural level, as a place where people go to discuss things. This exact interaction seems typical of what the platform tends toward: take smart people, reward them for making off the cuff quips, watch them act stupid.