If I'm not mistaken, AC Milan and Roma were the only Serie A teams that took their preseason games seriously, the rest of the teams did not, perhaps because they did not see it as a game that would help them prepare for league games. Come also took the preseason games seriously, winning the three games they had played against Lille, Al Ahli, and Ajax, but this doesn't mean that they would perform well in the league.
The reason I see behind AC Milan and Roma for not performing well in the preseason wasn't because they are under performing but to take things lightly since the main season haven't started and the season is very long which there is so many obstacles like injuries, suspended of players and also red card offences. All this are inevitable which is why it's considered. Regarding Coma, they just came in last season and they have to make themselves known that why they are proven it harder.
However, I was shocked by Napoli performance, I almost viewed them based on their preseason results because they had been playing so bad, out of the four games they had played, Napoli won one, drew one, and lost two. They face Casertana FC of Serie C side, Conte will compete for the Champions League, I believe the preseason games will not affect their league performance, but they need to improve their games.
Napoli performance can not be judged based on their preseason performance, I'm much confident that they are reserving themselves for the main competitions because they have a lot to battle with. They could have performed very well but there are many factors that was considered which I respect that. They are champions and they will like to continue in the path even though they aware that it going to be difficult.