thats the big question but nobody knows the answer yet. hes undoubtedly one of the most talented players for his age but time will tell if he turns into a goal machine like CR. Messi is in another league to CR, Mbappe, anyone basically so unless Mbappe becomes the best goal scorer + one of the best playmakers on top, Messi level won't be reached (not just talking about assist stats here either)
I'd say R9 is the most talented youngster i've ever seen, Mbappe is more Henry levels. Rooney was also amazing as a teenager and never really fulfilled the potential he promised so its early to tell
when you compare Mbappe at the same age to Messi/Cristiano you have to take into account neither of those 2 were guaranteed starters and had to fight for their place, ofcourse both were promising players. Mbappe is the 2nd most expensive player ever and is pretty much 100% a starter - one could argue hes guaranteed because of his talent alone but i think price tag has to play a part also
Messi and CR, especially CR, really only took off at 22 or something, quite a few players were better at a young age than CR, but his career took an "unusual" turn at some point and just exploded from there.
Its not so easy to make comparisons yet.