Lewandowski was at Bayern Munich for 8 seasons, so it will be difficult for Sadio Mane to reach the game that Lewandowski gave Bayern Munich.
Let's take a look at Lewandowski's first season with Bayern Munich on a free transfer from Borussia Dortmund. He played 31 times and scored 17 goals, now Sadio Mane has only played 14 matches and scored 6 goals, he still has a chance to match Lewandowski's goal or even exceed it in his first season.
This is a comparable game in my opinion than comparing Lewandowski's contribution in his last season to Sadio Mane's in his first.
It seems too fast and the fans' expectations are high for the performance of Sadio Mane because still in the first season and have 15 matches only. I think Sadio Mane's performance isn't too bad, it's just that the fans' expectations are too big and it doesn't give Mane time and space to prove his capacity as the best player at the moment. From 14 appearances with Bayern Munich in the Bundesliga, Sadio Mane has managed to score 6 goals and 3 goals in the Champions League from 6 appearances.
I don't think bad performance with 9 goals scored at Bundesliga and Champion because comparison with Lewandowski still need adapted for the first season in Bayern Munich.