Pikachu will eat anything if you tell him to do so, and sometimes if you don't. Even the megaphone.
One of the transition screens shows the player's mother yelling at Pikachu, telling him to wash his hands, which are filthy.
One part of the credits shows Pikachu watching a group of Oddish running by a tree. Then a giant one runs past
Hey, Pikachu's confused.
Pikachu's reaction upon getting bad food in Ochre Woods. If you sent in all three correct ingredients but sent a fourth that doesn't go well with the dish, Pikachu will stumble around with swirly eyes. If you don't send in all three ingredients, resulting in Mystery Stew, Pikachu will be launched into the air and will fall back down face-first. It's even funnier given that any other Pokemon that eats either one will just cough, making Pikachu look like a drama queen in comparison.