I think the performance of a team in a particular match can be used to evaluate how improved they have become and winning an important match can also serve as a Moral booster to a team and I think the performance of Barcelona in that game shows somw sign of improvement even though they have not really been so good this season but winning the game to move to the second place in the league ahead of Girona can help them to also do better wheb they face Napoli at home in the champions league.
But you are describing the biggest problem that top clubs have that are in the midst of a bad season and can't solve it. Bayern Munich is in trouble at the moment, but does it mean that they don't play any good games at all? No, they do play good games sometimes, but one week later it's all trash again. That is why a 4-0 is good of course and Barcelona has shown some improvement which can be seen since they didn't lose a game in quite a while, but still they are very far away from where they want to be. They have good minutes in every game, high ball possession and strong control over the game. But I am sure it won't take long until they drop points again out of nowhere. Just remember that game they played against Villareal. Full of mistakes all the time while having some genius moments. That is what Barcelona is about this season.