From the Shirtpunch.com's interview with the artist:
"Question: What inspired this design?
Answer: The ever cuteness and innocence of Miyazaki's movie My Neighbor Totoro."
About Shirtpunch.com: It's simple; ShirtPunch is your home for limited edition, pop culture tees... but there's a twist! We only offer one shirt every 24-hours, and they're always only $10.