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.
Out of the UCL and couldn't make it to the Europa League spot and that's weigh bad news for Sevilla this season, they couldn't accumulate three points needed. Sevilla have suffered from the hands of inconsistent performance and their players giving nothing improving to boost their performance, other than finding good solid performance to enchanced their stats. Sevilla have failed to qualify for the next round of the UCL after falling out. They will have to fight for a spot in important positions on the La Liga table because that's the one way to stay put for another major tournaments.
Sevilla's downfall has started from the previous few season its just that in every season there are some things that favor them especially in the trophy race just like last season where they did not perform well and their consistency was very disturbed only they could get the trophy but this season they also did not have an increase in performance and in the end it was even worse than last season considering that this season they did not qualify for the Champions League and could not seek luck in the Europa League because they were only at the bottom of the standing.
Worse still in domestic competitions they also dont have anything worth bragging about as they are not in a good place and i doubt they can be so in this case they can only rely on the Copa del Rey when it comes to competitions and you can imagine its still very difficult.