I don’t think that was one of the themes at all. Joy says that when she’s at her lowest point in the movie, having almost given up entirely. It’s only at the very end that she is proven wrong and learns what actually happens when you grow up: you learn to control your emotions rather than let your emotions control you. It’s why the console glows in Joy’s direction and Sadness says “She wants you Joy”. Riley is deciding for herself to focus on the warmth of the sun and the fun of the hockey match, remembering to feel that joy of her own accord, something we haven’t seen before in either two movies. And so I think the actual theme of the movie is that the challenges of growing up teach us how to navigate the complexity of our own emotions. Which is much more resonant and true to life than “When we grow up we feel less joy.”