Barcelona aren't having the best of times in La Liga at the moment, but they finished top of their group in the Champions League with 12 points. They had tougher opponents like Shakhtar and Porto. Napoli only had Real Madrid. A dead Union Berlin and almost the same Braga. I think Barcelona have a better chance than Napoli.
I also think that the chances of winning the match are for Barcelona, but we also have to see how the Napoli team develops in January because their match will take place in February which means each team can make changes for the better before welcoming their important match in in February next year. Although if you look at their current level of strength, it is a little more convincing for Barcelona to win this match even though Napoli is the home team in that match.
Basically Barcelona still has a chance to win against Napoli in the last 16 but ya, even though Barcelona has a chance to win but I personally cannot consider the results they got in the group phase . Because anyway, I think the opponents Barcelona faced in the group phase were also easy because ya, at least Barcelona still has better quality than Shakhtar and Porto as you mentioned.
Therefore, I think it's natural that Barcelona won the group but still, there is no guarantee for Barcelona to be able to go further in this knockout phase . But ya, about for the match against Napoli who are not doing well at this moment, then at least Barcelona have a chance than Napoli . But still, for me the chance that Barcelona has to win is not so good either, because basically Napoli is also not a weak team at all.