This wasn't about putting a rocket into space or a car into orbit. This was a proof of concept test.
At the moment companies who need to put up satellites have a few (but not many) choices. All are hugely expensive and time consuming. Rockets are fired into space and discarded. This particular rocket can carry double the payload of its nearest rival at one third of the cost
and return most of the big bits safely to Earth. This is an enormous technological achievement.
Regarding the car. They also needed to show that this particular system could lift something into space and put it into orbit. The car is about double the size and weight of the average commercial satellite. Satellites and scientific instruments, "useful stuff" that could have been put on board cost millions. This was a test flight and they were half expecting it to explode on the launch pad. Nobody in their right mind would put anything that expensive onboard. The car was simply the proof of concept. Given that Musk has spent/raised hundreds of millions on this I think he can be allowed his little bit of "whimsy" as you put it.
Oh, and this ...