He's undoubtedly a great player, but I can't help but think he's stifling his development in Ligue 1.
He has the potential to be CR7-esque, he started in the PL then switched to La Liga, both good quality, hard leagues to compete in. Mbappe is thriving in Ligue 1, but I feel the quality of that league just isn't the same as PL, Bundesliga or La Liga. I'd put Serie A above Ligue 1. They certainly have some good players in there, but it...I don't know...doesn't seem much of a challenge for him.
PSG will dominate for years to come, that's a given, and with the team they've assembled it'll be a walk in the park. Champion's league is different, they look strong, and Mbappe looks like he can step it up in the CL.
My argument is whether he can consistently perform brilliantly in the CL whilst competing in a much less competitive Ligue 1 (given that Monaco have sold 80% of their team). Due to the fact that he plays for PSG, it'll probably look like that, but if you isolate his stats, would it be much different from anyone else?
I dunno, I guess I'm trying to say that he's an awesome player with unreal potential, but I don't think he's 'all that' currently. He has something to prove.