I'm sure that Napoli lost this game on purpose. They are not in the best position in the Serie A standings and they need to keep playing in the Champions League. The Italian Cup is not important for them and they will not waste strength on it. The main thing for them is to improve their position in Serie A and try their luck in the Champions League.
If Napoli is still trying to improve its position in the Serie A standings, of course Napoli must always consider winning and it would be very unreasonable if Napoli deliberately lost the Coppa Italia to a team lower than them. Apart from that, Napoli also cannot give an excuse if they deliberately lost just because they are still in the Champions League this season, because their fans will certainly be very disappointed if Napoli admits that they deliberately lost a certain match even though it was outside of the Champions League competition.
I don agree with @fzkto on the view that Napoli lost the game on purpose because I don't think they will loose matches on purpose in such manner because of the champions league that's still week ahead, they will have enough time to rest before the champions league, so they will definitely not have to lose their serie A game for UCL games in such manner.
For a team that is considered lower than Napoli in the serie A I don't think Napoli will in any way lose to them intentionally it was just a circumstances that's usual with football taking place, there have always been times where weak teams are able to win big teams and that doesn't mean the bigger team intentionally lost, this may happen due to some circumstances some may result from the players performance that day while some maybe they the team just had a bad day or some times technicalities or poor execution of strategy.