The roadster (I assume you mean Tesla's roadster) is in orbit of the sun (ie. It has escaped the earth gravitational pull). Unlike the space debris which orbits the earth. Therefore the debris is a threat and the roadster is not.
It's around Mars. Is not a threat, but surely is trash... thereis any reason to do it unless a wierd marketing for people that didn't care about throwing trash for no purpose around.
It is in a Sun-centered orbit. It never had the fuel/capacity to do a retrograde burn in order to be captured by the Martian gravitational field.
It was necessary to test the rocket with a payload. The usual test payload is a concrete block or something comparable. They wanted to do something more fun. The major debris problem is all the stuff we have put in orbit around Earth, not stuff farther out.