Well, my description of overrated is because he is compared to Messi, CR7, Ronaldo Nazario, Pelé.He did more at his current age than Messi and Ronaldo combined. Any level, any stat. Doesn't mean he's gonna be as good as them, but he's not even close to be overrated. Monster of a player at that age. MONSTER.
People who think he is good because he is playing at Ligue 1 should look at his stats in Champions League, World Cup etc..
He'll never play for City unfortunately.
Players don't necessarily have linear progression. Some are great early and then drop or just don't improve, some are average early and bloom in their later years, some are just great and keep getting greater.
I hope the last case will be Mbappé future, with no big injury and with PSG. But time will tell how high he can go.