The good thing about Mbappe is that he is only 23 years old, he has a whole football career ahead of him, while Cristiano and Messi will never play in a World Cup again. In Cristiano's case, I don't know how the World Cup will have affected him, with the ego he has. At the end of the day, we can say that Messi is a better player than Cristiano, and some say he is the best player of all time.
Yesterday Lionel Messi successfully fulfilled his long time ambition and became one of the greatest footballers in the world. But this world cup also launched a youngster Kylian Mbappe to a path of greatness. The second goal in that finals was classic, this wonderkid didn't let the ball to drop before he gave it an outstanding finish. With Cristiano Ronaldo and Messi leaving the the global football stage in few years, the next side attraction to global football would be Mbappe. He is confidence, fearless and bold at just 23 years. But his only undoing might be his immature character challenges that sometimes make him unhealthily competitive and temperamental.
Messi is not better than Ronaldo because he led Argentina to a world cup victory while Ronaldo was unfortunate not to have achieved that feat. Both of them are great players in the same class. There are some indicators that might prove that Ronaldo is also better than Ronaldo. However, it is better not to discuss this issue because it can lead to a long and sometimes protractive argument.