Performance on a NT has been important in the past, for reasons that are now outdated. It used to be that national team competitions were, by far, the pinnacle of football. Good nationals teams were miles better than any club. So if someone dominated in club football but did not do well for his national team, you actually had NO evidence that that player was capable of dominating top opposition. So you could not truly call that player the best. While that metric still suffered from a really low sample size (not to mention that some players are from countries with bad teams so they have no chance), it really was the ONLY way of measuring how a player did against the best of the best. So people had to forgive the massive flaws in using it as a metric.
But now it is completely different. The Champions League is the best competition in the world. Top CL teams would wipe the floor with top national teams. How one competes in the CL is now better evidence of how someone does against top opposition. And the CL does not suffer from the giant flaws that using NT performance does. There is a much larger sample size of matches, since the tournament involves more matches and is played every year. And it doesn't matter what country you are actually from; if you are a great player, you WILL end up on a team competing in the CL. Simply put, the Champions League is OBVIOUSLY a better way of measuring how good a player is than NT performance.
Of course, NT performance also matters, simply because everything matters. Even performance in the Copa Del Rey still matters. And tournaments like the World Cup have an added significance to the public. But it doesn't really show us all that much about a footballer's quality. For instance, we wouldn't say that the Germany defense is actually good enough to shut Messi down. We know from the CL that those same defenders have gotten destroyed by Messi (prime example: the last Bayern match). And we wouldn't say that the WC final shows Messi chokes in big matches. We know that he has scored in two CL finals and is the top scorer in El Clasico. So realistically, the WC final simply shows us how a player played in ONE important, somewhat-high-quality match. But we have so many more data points now of important, high-quality matches that the WC final is simply not as important a piece of information.