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.
There are many teams that perform well against strong teams and perform poorly against less strong teams. Sometimes strong teams do not know the past analysis of weak teams and their players in detail so during training, the match situation goes against them due to not knowing the strengths of the opposing team players. If they were aware of other strong teams and their players in advance and practices their strategies to counter them in advance it would be easier to play against them. The same situation was seen in the case of Barcelona, they performed poorly against Rayo Vallecano. But despite being a weak team played very well because the Vallecano players were aware of Barca strengths and went to the field after studying the players of the opposing team.