In fact, PSG fans booed Messi at the beginning of the game when the squads were announced, so it’s quite reasonable behavior.
In general, I can understand them - the arrival of Messi led to the fact that PSG began to relegate from the French Cup and the Champions League in the 1/8 finals for two years in a row, and Messi himself disappears in important games.
From such a promoted footballer, they expected exactly more.
That is a risk that must be borne by star players like Lionel Messi, when they get what they want they will flatter it, but when expectations are not what they want they will scorn it. This has actually been felt by his rival, namely Cristiano Ronaldo. At the end of his career it seems they get the same treatment in the team he defends, such as Messi at PSG and Ronaldo at MU.
I don't think there is anything in common here. At United, Ronaldo had a conflict with the head coach, but I did not hear that the fans or a significant part of the community expressed dissatisfaction with Ronaldo (except for the haters who are ready to use any reason to hurt the one because of whom they sleep badly

).
Messi has a different situation, firstly he plays badly, secondly it is aggravated by the fact that Argentina beat France in the World Cup final. But the main reason is the bad game of Messi.