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.
Indeed, Sevilla's circumstances this year are depressing. What's happening, in my opinion, is the result of issues that have been building for some time. Their performance was marginally better the previous season, but the breaks were evident. They seem to have been on a downhill trajectory, and this is finally striking them quite hard. It's a hard pill to take, losing out on a Europa League spot and being eliminated from the Champions League. It appears that they have a lot of ingrained issues, whether with strategy or management. Really, there have to be some major underlying problems before you can go from UCL qualifiers to this. They require a makeover, perhaps beginning with a new strategy for their game plans or perhaps a team reorganization. It's a difficult time to be a Sevilla supporter, but in my opinion, these difficult periods truly assess a team's character