It will be hard for Paris Saint-German to win the Uefa Champions League trophy. Paris Saint-German has built its team around Mbappe, and if Mbappe is not in form Paris Saint-German will not perform well.
It cannot be denied that Mbappe's influence is quite big for PSG and if Mbappe leaves in the transfer this January then I am very sure that PSG's progress in the UCL will not be long.
However even though Mbappe is still surviving doesn't mean he will be able to lift this team's performance alone, I think PSG is not worthy of becoming UCL champions this season because we have to look at the strength of the team as a whole and PSG does not deserve that, especially this season their performance is very good inconsistent.
Although PSG still has Mbappe in the squad but still, it will remain difficult for PSG to get good results in the Champion League. So yes, basically it's not about Mbappe but it looks like PSG doesn't have the right management to build the team even better. Because indeed, PSG has a lot of money and or has better finances but still, if PSG does not have good management in building the team then PSG will still be difficult to reach the Champions League final.
Therefore, I think it is possible that if for example Mbappe is sold or leaves it looks like PSG will be better because it will work as a whole team and not just rely on one player. At least, this is currently happening to Tottenham in the Premier League who previously kept Kane but after Kane was sold, at least Tottenham got better performance and results at this moment there in the Premier League.