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.