My wish to Mbappe is Good Luck, I hope he didn't just make a right move at the wrong time because the three straight performance is out of it and fans keep demanding very high expectations from him which is why it's seems like he isn't doing enough, though his performance also dropped generally before he even made a move to RM.
He also has an ongoing legal battle with PSG over his wages. I don't think it affects his performance on the field for Madrid though, whether they are playing in the Champions League or not. I took a quick look at his stats and it seems like the number is more or less the same over the least 3-4 seasons, so I'm not sure if his form is getting worse or we're over-scrutinizing him because he made a big move to Madrid. Granted he moves to a different league with different opponents, so it's hard to conclude anything from the numbers. I'm pretty sure he wants to win the UCL though, whether that hinders his goalscoring ability or not is unknown for now.