skateboard riding at mall parking lot

There is no general rule about ownership: one parking lot I know is owned by the city, another is owned by the company that operates the mall, and in a third case it is owned by a third party who doesn't operate the mall. Either way, the owner of the parking lot has the property right to limit how it is used, and their agents (security guards, for example) can request that you refrain from skateboarding. They can evict you; they may not have the legal authority to physically toss you off the property, but they can probably perform a citizen's arrest for trespassing. None of this depends on how many cars are in the parking lot. It is more likely that a governmentally-owned parking lot will have a corresponding law restricting its use (whereas in the case of a private parking lot, restrictions center around general property law and the law of trespass).