Re: Douglas Wolk talks about this passage in his book Reading Comics
While it's very hard to say what Sim meant, especially given his later insanity, there's nothing about the treatment of this particular black joke that suggests we are supposed to have *contempt* for Po, as Volk insists. It doesn't deny his courage, even if it is quite ambiguous about whether there was any point to it.
I found it interesting that the book suggests that Po's scribbling survived to be printed. Though I suppose you could take the missing word on the final page shown to mean that it was merely Po's hallucination of same.