Real Madrid has some players that are good on their bench but they are not used most often by Anceloti to demonstrate their effort on the big games. He is using a set and doesn't make changes often, though it's good because what he aim is always achieved and what is the used of changing players when clearly he can win the game with the main squad that he has, perhaps the bench ones will used this opportunity to show him their best.
I quite respect the way Ancelotti manages his team, having the first 11 squads always in the forefront and preparing other good players as alternatives too. It shows how he understands the stamina of his squad and where and when to shuffle them. He's a coach that works with much calculations and always ready to take decisions while withstanding the demands that comes with working with such a top class team which sometimes suffer player injuries and instability in the understanding of the old and new players.
But jokes aside, I don't think it is fair at all for any of those players. Mbappe is also one of the best players in today's football. Every player can be criticized in some periods. I don't think he will feel any pressure over himself and have problems on focusing.
But he would get better in time by adapting.