Far too many amateurs confuse RF and DC grounds, which leads to misunderstandings about the differences in DC ground and groundplane.
At low frequencies, a vehicle represents a capacitance to ground, not a groundplane. The losses involved are referred to as ground losses.
As the frequency is raised, the vehicle starts acting more like a groundplane. At VHF frequencies, it can act easily as a groundplane if the mounting scheme is sufficient.
A mag mount may indeed be DC grounded which doesn't mean a thing with respect to RF ground. A mag mount capacitively couples to the groundplane (in this case the vehicle under it), just like a vehicle capacitively couples to the ground at HF frequencies and adds losses.
The worse aspect to mag mounts has nothing to do with their additional ground losses however. It's the safety concerns should you have a crash. There has already been one reported case of a death caused by one, and most vehicle insurance plans do not cover such a loss.