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.
This season Messi has only scored four goals in Ligue 1, the tally Messi has scored far behind team-mates Neymar and Mbappe. Neymar has scored 8 goals, while Mbappe has contributed 7 goals for PSG. Messi declining number of goals is not because he is not able to perform well with PSG, Messi contribution to PSG is very good, he has contributed 7 assists this season. Messi really enjoys his new role as a playmaker, this role is very suitable for Messi because he can't run like he used to due to his age factor.