The movie follows a girl named Lillian, who, on a whim, decides to run away and embarks on a journey across America. Along the way, she encounters a series of extreme characters, each representing different facets of society.
Lillian to written to be as apathetic, opinionated, and jaded and for most the movie she remains this way. So I think Lillian serves as a lens through which we observe these encounters, not an actual driver of the plot simply there.
Interestingly, she remains largely unchanged by the experiences she goes through, which for me raises questions about her character's depth. While the diverse individuals she meets have profound stories and perspectives, Lillian's lack of development can make it feel like we're missing out on deeper reflections on these interactions.
This is why the story might have benefited from being a series rather than a standalone film. In a show format, Lillian could have had the space to reflect on her experiences, allowing her character to evolve in response to the people she meets. She doesn't have to change as a person because I think how she was written is also central to the message of the movie but there simply isn't enough breathing room in the movie, and there would have been some wonderfully emotional scenes if there was just the chance to.