Legal Question in Administrative Law in California

Yes, it is at least a civil wrong addressable by the car owner via a suit for "trespass to chattel" which is a baby brother tort to the commoner and more significant tort of trespass to land. When most people talk about trespassing, they are thinking about walking across or otherwise invading someone else's rights in land. However, it is also a trespass when you fiddle with someone else's belongings other than land, whether it be the family pooch, their car, or whatever. Generally only nominal damages are awarded for things of the sort you ask about, but the answer to your question is indeed yes. There may also be local ordinances about handbilling connected with anti-litter concerns. Finally, some placing of handbills is done on private property such as shopping center parking lots where there will also be a possible trespass to land issue.