I think it is rather the exception than the rule that a team which is performing bad in the domestic league is one of the strongest teams in the Champions League. Yes I know that now some people will come up with examples, like Sevilla and how they won the Europa League last season when they were so bad in La Liga. But where is Sevilla now? They finished their Champions League group with 2 points out of 6 games and got destroyed in almost every game. There is a correlation between the overall performance of a team in the league and the other competitions, but yes there are exceptions of course.
Some teams have poor performance in domestic league but success with quit well performance in Champion League, difference with Sevilla performance in this season not only poor in La Liga but also failed and finish on bottom standings in Champion League group phase. But another teams Napoli have quit well performance in Champion League and success qualify to knock out round although with 2nd standings finish position but they have poor and inconsistency performance in domestic league.
I think domestic league and Champion League have difference atmosphere and some teams focus with quit well performance in Champion League but they have poor performance when playing in domestic league.
Napoli was playing in a pretty weak Champions League group and it was not surprising that an average performance would be enough for them to qualify for the knock out rounds in the Champions League. If you call their performance in the Champions League a quite good performance, I don't know but we do differ a lot in opinion here. Their performance was not that good, but good for them that there was no team which was really able to pose a threat to them. If they ended up in a more difficult group, they would have probably failed to qualify for the next round. This time it was a happy ending for them.
It's not really weak like you said above. Given that Napoli was competing against a team as strong as Real Madrid, it would be inappropriate to criticize their performance as weak. I have opposite opinion about that and it feels like napoli is declining after it has done all of matches in the group stage and qualify for the next stage. Napoli is not as good as last season but it can still compete to face barcelona soon. To put all of the players into the defensive line, the only thing that needs to be done by Mazzarri is to make the necessary adjustments. The club shall take counter attack as opportunity to beat barcelona.
Barcelona was a club that is focus into how to build more ball possession. If Napoli focuses on playing with a strong defensive strategy against Barcelona and utilizes effective parking of the bus, they can significantly increase their chances of beating them easily. Napoli's group considerably placed at the medium group.
It has real madrid as the main competitior and do you forget that if barcelona got easy group compared to the napoli? The way to beat barcelona to go through the fully defensive line like parking bus strategy.
It will be making barceloan frustrated with it. Barcelona is going to lose its concentration when the club was always trying to pressuring napoli.
I didn't say that their performance in the Champions League group was weak, I said that the group was pretty weak. Since the top two qualify for the next round, having Real Madrid in your group is not a problem since the second also advances to the next round. But Union Berlin and Braga are both teams that are far weaker than Napoli and that is what I meant. If Napoli had played in the group where PSG, Dortmund, Milan and Newcastle played, I don't know if they would still be around in the Champions League. Their performance in the group stage was ok, it wasn't good. But now we will see how good they are when they finally meet some of the better teams although Barcelona isn't good either.