Thin by modern standards, but it's my understanding that the Roman administrative system set it quite apart from many of the other polities it interacted with. Certainly the post-Roman West.

Thin layer? No, the core reason for Rome's success was the efficiency and adaptability of this administration. It could always generate much more taxes, soldiers, loyalty and diplomatic influence than other states that had the similar amounts of land and people. Then, when something had to change, the elites or even the people just deposed the emperor and got someone in power that did what was necessary.

I think the "thin" qualifier here was to mean that it wasn't a deeply cultural and nationalistic embedding of a nation as what we think of national today. It was more akin to modern colonialism where a certain degree of exploitation be the colonial powers was what roman authorities cared about but the degree of cultural influence depended much on the leeway allowed by the strength of the pre-existing culture of the conquered place