"I mean he said he was aiming for Mars but then ended up in the Astroid belt"
In all honestly, he was aiming for AT LEAST Mars. They burned the upper stage to depletion regardless of target. It showed them how far the upper stage could get almost to Ceres, which is no mean feat. Honestly it would not have taken all that much fuel to get to Jupiter's orbit. In fact, the orbit it is in now will actually be affected by Jupiter making the orbit somewhat unstable, and the possibility exists that the roadster will be now ejected from the Solar System entirely by Jupiter at some stage in the distant future.