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.
What you say doesn't make sense, it's impossible for a team to deliberately lose a match, let alone a big loss against a weak team in front of its own supporters, which is quite embarrassing.
Indeed, basically Napoli's performance is very bad and inconsistent at the moment so they suffered this embarrassing defeat.
There is no other way for Napoli other than to recover and start improving, but with their current new coach and the few matches they have played, it appears that the possibility of them being able to compete at the top of Serie A this season is very small.