when expectation is too high on a player, it makes it deficult to see a good performance from him as he will have to play under too much pressure. His coming into the Madrid squad gave fans that Ronaldo's vibes and most pundit expected to see madries defense as the toughest that can ever be across leagues. The reality strucked And he's unable to perform to the level fans expected of him and seeing how the team seems to be struggling since his addition to the club, it's only normal to see statement like this from fans. But then, it's just an opinion that doesn't affect the teams decision on who is to play a certain game at a particular point in time. The line up for the game against osasuna is already out and he's on the line up together with vini, Rodrygo and Bellingham. Hopefully, it will be a win and he gets to give an impressive performance just so the frustration can reduce on himself and fans.
People are not happy that Mbappe joined anymore because they have seen what he has done so far. They were excited because they thought he would score bunch of goals, but he is not and that is why they are disappointed in him. Mbappe has 6 goals in the league so far, by comparisons Lewandowski is like 35 years old or something and he has 14 goals so far. This is the reason, we are seeing something this different because Mbappe has played terrible all year long.
And maybe it is not just all his fault, maybe it's because Mbappe joined Real Madrid to be a left winger, or play where he feels comfortable and he is played out of position which makes it hard for him. I have no idea whose fault this is, but fans are not happy with what they are seeing so far.
Plus, it is not even just about stats, like you could have decent stats, assume he scored more, but if your teammates are giving you great passes, and you are not turning them to goals, then what's the point of scoring once or twice a game, when you are missing 2-3 more open wide shots? That is the situation, he is not scoring enough at all, but on top of that he finds a ton of chances thanks to his teammates and he is still missing those shots.