The PSG team has been trying hard for the past few seasons to build the team to secure the Champions League title. But despite having the best players in the world in the squad, the team failed to perform well in the Champions League. I don't think PSG team can perform well in any league other than League One.
I believe it is more complex because PSG could have played better if they had more of those hard challenges in their league. They lack challenges, they can't get used to the hardest competition mode during their Ligue 1 games. Maybe a few times a year they have a game that really challenges them in Ligue, but is it like really hard? No and that is a problem whereas Manchester City is playing tough games almost every second weekend including the Champions League. They are used to working at their limits. PSG is not.
Every league has a different tension and with that, it doesn't mean PSG doesn't have tough competition in Ligue 1. Because anyway, if PSG does not have tough competition in Ligue 1 then of course PSG is very easy to dominate Ligue 1. But the fact is, PSG still has to work hard to dominate Ligue 1 and that means, PSG still gets tough competition in Ligue 1 too. After all, you can't think Ligue 1 is an easy league, because after all the tough competition will always be there even if they dominate the league and it's never easy to achieve the highest achievements.