I mean, I don't mind seesing a film to have a lack of antagonist. But, I still don't see why. For all I know, every film show have a conflict, or a person showing some antagonisim to other characters.
I do believe that the first Toy Story shouldn't need a villian because it's supposed to be about two toys learning to understand and to get along with each other. And in Finding Nemo, Darla only shows antagonisim by killing her pet fishes by shaking them in her fish bags.
Some or few of these Pixar films don't really need to have any villans. Despite the fact this paragraph being written in the Inside Out page trivia.:
"This is the first Pixar film to lack a concrete main antagonist. Anger, Jangles, and the Forgetters are the closest the film gets to having an antagonist, and even Joy herself shows shades of antagonism towards Sadness, but other than that, the film has no main villain."
Possibly the villian in this whole story is suppose to be the changes going around and the way a person deals with the lack of control around them. Riley feels like she losing control of everything around her, even her own emotions, but in the end the changes stop bothering so much and she finds a new way to feel in control of her life again.