I think there is a lot of Juventini in the Campania region, but Napoli ifself is quite parochial IMO. I think there's obviously a decent presence of Juve fans (there is all over Italy) but Napoli being the most successful and popular club in the South (other than us lol) means they have plenty of fans in their own city.
Oh and their reasons for hating us are beyond football. It's the whole North V South, Bourbon V Savoy thing.
The Kingdom of Sardinia/Turin defeated and annexed the Kingdom of Naples to unite Italy in the 1860s. And their fans are still butthurt and convinced that they'd be better off without the top half of Italy