Mbappe played for PSG for a long time but he could not win any major tournament, that is why he decided to come to Real Madrid last season and he thought that if he went to Real Madrid, he might taste the Champions League trophy in his first season but in reality that did not happen. PSG, which Mbappe left to join Real Madrid, qualified for the Champions League final this season and they have a good chance of winning the final match.
If PSG can win the Champions League for the first time this season, then Mbappe may regret it a little and may think that if he had stayed at PSG for another season, he might have won the Champions League trophy for the first time. However, it can be said for sure that he will have the most success in the club he is currently at.
Maybe the first season didn't go well, but the coming seasons will definitely go well for Real Madrid and Mbappe will have a good chance of winning the Champions League title for Real Madrid next season.
He still has a high chance of winning it at Real Madrid in the coming seasons. All Real Madrid have to do is get their defense right and maybe part of the midfield, and they will be good to go in Europe. Meanwhile, should PSG lose to Inter Milan, chances of the getting back to the Final in the coming years may be low.
I don't think he will regret so much. Time is still on his side and of course he's getting huge wages!