It doesn't make sense for Napoli to deliberately lose a competition they still might win, to me it's just an absurd assumption. Napoli have experienced a very significant drop in form this season after winning the Scudetto last season, so they need to take advantage of every competition to win it instead of letting other teams beat them easily.
Yes, this is a realistic thing. It will be very difficult for Napoli to reach the top of Serie A this season and defend the title they won last season. Not only is their condition quite poor, but the distance between points is far away and the strength of the opposing team is actually getting better this season. The competition is very tough this season. And what's more, at the moment, I'm sure Mazzarri's focus is on improving the performance of his squad and trying not to fall into an even worse position. Because after all, Mazzarri also has quite a tough challenge here. At least if Napoli can still enter the UCL again next season, this will be enough for now.