Any moment where you play the Poke Flute within a Pikachu's earshot.
"Pi-ka-CHUU~!!" (uses Thundershock)
Meowth getting hit with Pidgey's gust.
Dancing Meowth, which, of course, requires the Poke Flute.
Not necessarily—if you have Pester Balls, simply save Meowth from the vengeful Pidgeys and he'll start dancing on the spot.
You can find a sleeping Snorlax on the Beach stage. Simply taking a photo won't yield any satisfactory resutls, but hit it with a Pester Ball and it'll get up for a few seconds. Use the Poké Flute, and it will dance!
The first Pikachu you see in the Tunnel stage. Take its picture, and it'll run off until it hops on the first Electrode. And then Electrode explodes with Pikachu unharmed, but startled.
The Volcano stage has a group of Charmander yelling "Char!" over and over. Michael Haigney's voice especially makes it funny.
Throw an apple right between two Magmar, and they will fight over it, knocking each other out with fire breath.
The River stage is a cornucopia of comedy. We have Bulbasaur's "Bul-ba!" when it gets hit with a Pester Ball, the Slowpoke, and the funniest part of the stage is Psyduck. If you hit the yellow ducky with an apple or a Pester Ball, it sinks into the water, but it'll continuously pop out of the water and quack.