Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Amused
Don't get us wrong: it's not that all (or even most) of the situations in Pudd'nhead Wilson are amusing. In fact, some are downright horrifying.
The prospect of being forever separated from a loved one because of slavery, for instance, is hardly a knee-slapper. Nevertheless, many of the novel's depictions of events and characters suggest that the narrator is pretty amused by a lot of what goes on in Dawson's Landing.
Take the scene in which Percy Driscoll chews out his slaves after he finds some money missing and threatens to sell them all down the river. Percy booms:
"[. . . ] I will not only sell all four of you, but I will sell you DOWN THE RIVER!" (2.36)
Writing in all caps—really, dude? You're hurting our ears.
Anyway, this is some pretty serious stuff, no doubt. But look at how the narrator describes Driscoll's reaction after he decides not to sell the slaves down the river after all:
He knew, himself, that he had done a noble and gracious thing, and was privately well pleased with his magnanimity; and that night he set the incident down in his diary, so that his son might read it in after years, and be thereby moved to deeds of gentleness and humanity himself. (2.42)
Okay, we've just seen that this dude is anything but gentle, noble, or humane, scaring those poor slaves to death and flaunting his power to sell them at his whim. However, he thinks that his decision to sell them locally rather than down the river is about to put him in the running for the Nobel Peace Prize. And—voila—we can't help but be amused at how ridiculous his totally distorted impression of himself is.
Now can't you just picture the narrator struggling to describe Percy writing in his diary without totally cracking up?
Sure, some might say getting amusement at the expense of others isn't nice. But let's face it: based on their actions and attitudes, some of these characters may deserve it.