Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Hilarious, Warm, Condescending
For the most part, the tone of Pnin is hilarious and warm. It sort of feels like your corny uncle is telling jokes most of the time. So you get things like this: "On the third hand (these mental states sprout additional forelimbs all the time)" (1.12). Pretty funny, kind of cheesy, and not too malicious.
But as often seems to be the case with Pnin, that's just the shiny face of our narrator. Below that we see a narrator that likes to take a shot at Pnin every chance he gets. So almost everything ends up sounding condescending.
For example: "Dr. Pnin nimbly walked into the passage, voiced a query, received a quiet answer, and returned with his son Timofey, a thirteen-year-old gimnazist (classical school pupil) in his gimnazicheskiy uniform—black blouse, black pants, shiny black belt (I attended a more liberal school where we wore what we liked)" (7.1.6). Yes, of course he attended a more liberal school where he could wear whatever he wanted. Show off.
The one-upmanship is actually almost compulsive. When the narrator describes Pnin's simple toy airplane he just has to mention how much more awesome his own was. He says: "I had a similar one but twice bigger, bought in Biarritz. After one had wound up the propeller for some time, the rubber would change its manner of twist and develop fascinating thick whorls which predicted the end of its tether" (7.1.6). We're sure it was also made out of gold.
Altogether, this tone attempts to lull us into security, and then unsteadies us when it shows its negative side. And we don't know about you, but that makes us feel pretty uncertain about everything the narrator says.