The note "made on Earth by humans" is written on the car's circuit board, so it isn't prominently displayed, but it might suffice as a friendly greeting to an extraterrestial.
In addition to the Roadster, the Falcon Heavy's upper stage is ferrying the car's dummy operator, appropriately named "Starman" by SpaceX. The company streamed the first four hours of the rocket's journey, with Starman behind the wheel as the Roadster drifts away from terra firma. Words of encouragement are there for the space-journeyman, as the message "DON'T PANIC!" is scrawled on the car's dashboard. Starman appears to have nerves of steel, though, so the message might fail to resonate.
SpaceX's video is admittedly very long, but the footage is breathtaking at times.