Of course there is no team "necessarily" reaching the final because of their performances in the domestic league, I would never say that. I would still argue that it can be an indicator, but there are extreme exceptions like we have seen with Sevilla in the Europa League. But I am quite sure that if we look at all title winners from the past, the majority that won international titles also played a strong season in their home leagues. I said the majority, before anyone says that I said something like "every time".
This unpredictability factor is always inherent in these international competitions as players can show different performances out of a sudden.
As one indicator, it may be true that performances in the domestic league can determine how a team can compete for the Champions League title. That was aimed at by Manchester City last season and they are so strong in the domestic league and the quality of players they have is almost universal. But talking about the Champions League is not enough about their strength in the domestic league because other indicators are also needed as you said because the Champions League is quite different from other leagues.
Sevilla may be different because they are taking part in a competition that is not as competitive as the Champions League and there also does not have a strong team when they enter the final. So the team strength is much more balanced and really depends on how the coach is able to play his role in implementing strategy.
I also find it difficult to say that one of the top teams played a bad Champions League season when they play against each other in the round of 16 or quarterfinals. I mean if it is Real Madrid vs. Manchester City in the quarterfinals, one of those two teams inevitably has to leave the competition, but does it mean that the loser also played a bad Champions League season? One team has to go, so a lot depends on the drawings as well.
Apart from how the coach implements the strategy, he may also see opportunities and if he has entered the semi-finals or final, the decision is entirely up to the coach and the quality of the players. When they are able to take advantage of the slightest opportunity, they will come out as winners. There are many indicators that we can look at and maybe everything is never the same every season. Football relies on strategy, the quality of players and finally luck when they are able to take advantage of existing opportunities then this opportunity will be a sweet result for the club.