In Chapter 5 of the novel, Twain's narrator, Huck Finn, presents an episode with Pap Finn using dead-pan humor. This type of humor refrains from making judgement or commentary upon activity that is clearly comical, instead opting to be matter-of-fact.
As the events of the episode are clearly comical yet the depiction of them is matter-of-fact, we can describe the comic technique as ironic, sarcastic and dead-pan. Dead-pan humor is not absolutely always ironic, but in this case it is. (Whenever there are two or more layers of meaning in a literary text, that text is presenting an example irony.)
In this chapter, Huck's restrained response to and depiction of Pap's actions suggest that Pap's behavior is normal. Yet, because it is extreme and odd, this behavior is not normal. The discrepency between these two views creates humor.