This Character Had The Worst Story Arc On The Good Place
Although The Good Place is defined by the growth that its characters go through over the course of the show's run, that growth is not applied evenly across the board. Some characters, like Michael (Ted Danson) and Eleanor (Kristen Bell), change immensely over the course of the show's run. Michael goes from hating people to deciding that he wants to be human, and Eleanor goes from a truly terrible person to someone who cares deeply about others, even when she doesn't know them personally.
Other characters on the show certainly change and develop, but they don't necessarily get the same terrific arcs that the show's leads are given. Characters like Tahani (Jameela Jamil) and Chidi (William Jackson Harper) grow, and they become much better people as they learn to overcome the baggage they had as people, but on a show filled with great arcs, they aren't necessarily the very best. They aren't the worst either, though, because that spot is reserved for someone else in the main cast.
Jason is hilarious, but relatively static
There are plenty of reasons to love Jason Mendoza (Manny Jacinto). He starts the series by pretending to be a silent monk, and we eventually discover that he's a lovable idiot from Jacksonville, Florida, who only wound up in the Bad Place because of his love for petty crime and blowing things up. As we see Jason grow, it's clear that he's becoming a better, more conscientious person. He remains very far from The Good Place's smartest character, but that idiocy still allows him to care for his friends and always work toward doing the right thing.
Fundamentally, though, Jason is not the emotional center of the story. He's designed primarily to provide comedic relief from the sometimes complex philosophical debates that the show focuses on. Because Jason is often sidelined in those conversations, he doesn't get a chance to grow and change as much as the others. Although he goes through plenty of change, he's the most static of the show's central human characters.
Jason's lack of an enormous, moving arc doesn't make him any less fun to watch, and Jacinto is an expert at providing goofy energy whenever he's called upon; his character has some of the show's very best jokes. His arc may be lacking, but The Good Place still allows everyone's favorite Jacksonville Jaguars fan plenty of time to shine over the course of the show's run.