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.
It's hard to say that because we've seen several teams perform extraordinary in the UCL when their performance in their domestic league was very poor, but that only happened to a handful of teams and not forever so we can't draw any conclusions about that, what is certain is that the teams that have Good performance in their domestic league has a much greater percentage or opportunity for them to be able to do the same in the Champions League competition.