I can't say Sadio Mane is a flop now, but he still hasn't found the game he's used to playing like at Liverpool. Bayern Munich still haven't found the chemistry between their new signings. I think he still needs time for that, to get the best out of him. Even though it takes time, I'm sure Mane can show something that is his strength. The league has only started a few weeks, it's not as expected but we have to be a little patient to see Mane.
Now is not the time to conclude that, because all the new players have not played so long this season in their respective teams. And also each new player will need a different time to adjust to the old players in the team they occupy. For example like Mane where he hasn't fully adapted to the Bayern Munich team this season as you said and I think that's true. So it would not be complete if we had to conclude now for every achievement of all new players including Mane.
Mane will adapt,and he will become one of the best players at Bayern,it's all time,he was doing very well at Liverpool,and he has alot of task to fulfill because responsiblities will be on him to fill the void left by Lewandowski.We all know it will really be difficult to replace Lewandowski,but with time,even if he cannot do exactly what Lewa did at Bayern,he should atleast be able to score a number of goals that will make the fans happy.
Another thing is that he hasn't really blended well to the style of play of Bayern,while Lewandowski has really hit the ground running at Barca,and he is one of the leading scorers already at the league.But I know with time,Mane will do well.