I had initially hoped Napoli would be in the same group as Liverpool. I would have liked to see a battle between both teams, who have a similar style imo. I'm not a fan of Klopp though, and even with a squad worth €100 million more in market value according to transfermarkt, i think Liverpool would get exposed against a side like Napoli. I think Napoli are smarter and more cunning than Liverpool, and have a better defense as well.
But barring an off-day for City, i don't think Napoli can win this. City has too much attacking force, and Napoli doesn't have someone like De Bruyne to dictate play.
I follow Mertens as much as i can (he's from the same city i'm from) and while he can be a bit nasty at times, i would never call him a "diving bastard". Sure, he's only 1.69m tall and weighs maybe 60kg (if that), so he just goes down easily in physical duels, and he will claim a foul when that happens (what attacker wouldn't), but that doesn't mean he dives on purpose without contact. I've also never seen/heard him being described as such.