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.
Paris Saint-German has really been a great team but they were never lucky when they play in the champions league and got eliminated all the time even the times they reached the finals, they were not able to lift the trophy. Mbappe has moved on and the pay cannot be used to judge how he’ll feel or how people will feel about him. He has done his own part during his stay in the club, and all the moments he spent there were worthwhile which contributed to Real Madrid wanting him to play for them. Mbappe will still win the champions league with Real Madrid, it’s not too late for him to achieve that. The champions league if won by PSG means it was not destined for Mbappe to win it at PSG but rather with another club which is looking it that he’ll win the champions league sooner than expected.