Barcelona dropping points against weaker teams is not new. We have seen this many times in the past. Even in the previous season, Barcelona dropped points against weaker teams in several matches. On the other hand, we have seen Barcelona perform well against teams like Real Madrid, Atletico and Sociedad. Barcelona failed to win against Rayo Vallecano. There is no denying that Rayo's performance was incredible. They definitely earned points in this match. At the same time, I would like to say that if Hansi Flick wants to dominate La Liga, then every match will have to be considered very seriously. If he cannot continue his winning streak, it will not be possible for Barcelona to dominate La Liga.
You are right that Barcelona has history of struggling against weaker teams while playing well against strong rivals. However last match with Rayo Vallecano was 1-1 draw not loss for Barcelona. Your point about Hansi Flick needing to keep winning streak is very important because if Barcelona wants to win league they can not afford to lose points to any team. Flick challenge will be to make sure his team plays consistently well in every match.