I can confirm that Nico di Angelo would have been part of the Italian nobility ~☆
For those following my streams of consciousness, they know I am planning to write a fanfiction about Maria di Angelo and Hades, a retelling of their story in an entirely Italian context.
Currently, I am researching, or rather reviewing, the history of my country and the poets who have been part of it. If I manage to write everything I have in mind, it would make me immensely happy
However ~~ the reason I say Nico would have been a noble is very simple. In canon, it's said that Maria's father was a diplomat.
Now, diplomats in Italy in the 1900s were cultured, educated individuals, commonly noble people. Nico's grandfather is a nineteenth-century man; in 1800s Italy, nobility was at its peak, only later did the bourgeois classes emerge, the classes of wealthy merchants commonly called "new money" and these people, during wars, such as the First World War, financed the armies, but noble men, like Nico's grandfather, engaged in diplomacy and worked behind the scenes of the war.
So Nico is not only the Ghost King but also a Marquess in Italy [even though titles in Italy have been defunct for decades but still]