Attitude aside, I think it's always been clear that he's not all that. A very good player who was always very mature for his age but IMO he never showed the talent that Neymar, Cristiano, Messi or even Hazard possessed. Players who have great physique, decision making and end product but some technical deficincies tend to develop not that much after that since they already have the attributes associated with experience. Improving the motoric aspects of the game is far harder after a certain age. And Mbappe always had a bit of a fluky technique in certain situations. That's of course criticism on a very high level since his also elite in the technical aspects of the game but not to the same extent as the "wonderkid generations" that preceeded him. The comparisons with young R9 were always over the top.
That being said some of the reactions here seem way over the top. He's still a world class player and can become one of the greats, especially if he gets his attitude back on track. And you won't find many 22 year olds as good as him. The Rashford comparisons are a bit silly from an outside perspective. No dig on Rashford, but he would already exceed expectations if he some day reached the level Mbappe already has despite being younger than him.