Well they did not actually physically damage the car, so it is not vandalism as such. But artistic? I think not. A bunch of kids scrawling their own images of greater or lesser imagination and creativity is more anarchy than art. It's only "art" in as far as any unstructured, random, incoherent assemblage of visual images is art. It is only as much art as the typing of the mythical chimpanzees trying though infinite random presses on the keyboard to recreate the works of Shakespeare is literature.
So there!