This is because Mbappe does not fit with the Real Madrid team, at PSG Mbappe can be a leader and he can even put his ego forward but at Real Madrid this cannot be done because Real Madrid is a big team that gathers top players so Mbappe must be able to adapt to that.
For now Mbappe is very burdened with the fans' taunts but he still has time to fix that, with the individual quality that Mbappe has I think he still has the possibility to be able to turn the current situation around.
Some how the leadership role on mbappe at PSG was a boost to his mind possibly, probably because he gets to take responsibility at some times but then when it has to do with the fact that at Madrid he's not getting such opportunities as there are some equals like him actually in the league. Madrid isn't a very good place that suits his ego somehow because he's got competition in the team and even the league generally, well I would still think he be given more time before we finally assume he's a burden and not a benefit to Madrid.