I think all of these are excuses. When it was said that Mbappe would come to Real Madrid, it was said that he could be the top scorer in La Liga. He has not shown his true performance yet, but last year was not very bright for Lewandowski either. Every player can have a bad season as well as a good season. Of course Mbappe is not in a bad season, but he could not show a great performance as everyone expected. I think he scored 2 goals from penalties.
Mbappe is a very talented scorer. He scored a lot of goals with PSG. So we all thought that since Real Madrid's squad is more strong and organized, also Real Madrid's midfield is very strong, Mbappe might be able to score more goals with this squad. But so far we haven't seen the expected performance from Mbappe.

Mbappe has scored 5 goals in 7 appearances in La Liga. Among them, he scored three goals from the penalty. And assisted only one.
On the other hand, Lewandowski scored 7 goals in 7 appearances. Scored a goal from Penty's chance. and made two assists. Mbappe is ahead of Lewandowski so far. I wouldn't be surprised at all if Lewandowski is the top scorer in La Liga this season.