I have little doubt that the animators at Pixar Studios didn't simply animate/model this writing after one of their own. It is (I would say) extremely unlikely that this is a font in itself.

During the movie, you actually see one of the characters in the movie writing out the word "Sunnyside" on the box. In essence, the word was drawn out from scratch.

You can even tell by the two distinct "n" letters. The first "n" has a wider space than the second one.


In short, this is not a font.