If PSG can win the Champions League title, it will be the most embarrassing for Mbappe. But it is an honor for Ligue 1. Everyone insults Ligue 1 as the farmer's league. Because Ligue 1 is not that competitive.
There are other good players like Messi and Neymar who left Paris Saint-Germain for another club, and Mbappé wasn't the first player to leave Paris Saint-Germain for another club, so I don't understand how Mbappé will be embarrassed if Paris Saint-Germain wins the UEFA Champions League trophy this season. Unless you aren't happy that Mbappé left Paris Saint-Germain for Real Madrid and you want Paris Saint-Germain to win the UEFA Champions League trophy so that Mbappé will regret why he ever moved to Real Madrid.
It might be more appropriate to regret his departure because if Mbappe had been a little more patient, it is likely that this season Mbappe could have felt the joy because PSG could reach the final and their chances of winning the UCL are quite large, but if Mbappe is still at PSG this season, it does not necessarily guarantee that PSG can reach the final. In fact, in the last few seasons Mbappe has been in the team, PSG has not been so impressive. Previously, many people underestimated what PSG could do without Mbappe, but in reality PSG can still run without the player and is even much stronger.