I wonder what indicators you used to determine the extent of Messi's influence on PSG's game? In fact he doesn't show anything above average - PSG dominate Ligue 1 and any attacking player in midfield will have a good goal/assist statistic. Even if you compare him directly with Lewandowski, I would say that Lewandowski shows a much better productivity in Barcelona (although again you have to wait for games against strong teams).
Messi is not the main striker in PSG now. So it is foolish to think that he will score a lot of goals. Galtier changed Messi's position. Now I think Messi's main responsibility is to strengthen the bonding between the midfield and forward players. Messi can now be considered a playmaker. And Messi has been able to adapt to the new position quite well. And I think we will see better performances from Messi in the future.
It is foolish to object to the fact that no one wrote. No one suggested comparing the goals of Lewandowski and Messi. The fact is that even if we are talking about Lewandowski and some goalkeeper, we can compare their contribution to the common cause. And the fact is that PSG are so strong in Ligue 1 that in Mbappe's place there could be a bunch of other strikers and have the same statistics as Mbappe, and this applies to Messi too. He does not show anything surprising in the center of the field in the position of the playmaker/under the strikers, the usual average game.
I think that the simple analogy is going to be that Lionel Messi is a talented player. However, right now his team does not require him to score a lot of goals. There are other players who can do that as well.
But in Barcelona Robert Lewandowski was brought in for the sole purpose of scoring a lot of goals and being the finisher of the team. And with the type of competition that PSG faces in Ligue 1, they don't need a lot of goal scorers in the team. But that does not mean Lionel Messi is not capable of scoring goals often. In his recent appearances with the national team Argentina, he has demonstrated that he is more than a capable goal scorer.