It's just Messi and Neymar's PR teams and fanbases teaming up and hating on the real star of the team who has been given a lot of power.
Mbappe probably has more intelligence than both Messi and Neymar put together(hardly an achievement tbf since both are as thick and colourless as they come and have the personality of a used tissue paper). Mbappe comes across as very driven and mature. He's a Ronaldo fanboy and wants to be the star man of his franchise(yeah I used the word, that's the ground reality of Football in 2022) and needs other players to work for it so that the club can get the best out of him geared towards his strengths just like Ronaldo and Messi had in their pomp. Nothing wrong with that whatsoever.
Neymar is an utter twat who does pranks and screams like a lil girl. Messi is an unnecessary addition only because of how big a brand he is. In PSG, you can't do anything to 3 players and that's just a death sentence.
Neymar needs to be sold. Mbappe HAS TO BE the main man with Messi feeding him for now while some young talented stars need to be signed creating a more balanced team.
The Barca fans calling out Mbappe's ego are the same people who blindly support a guy who asks for private boxes with another man in a stadium.
It's just the way how it is in professional Football. If you're at the top as a youngster you'll get insane amount of power.