What Nice and Monaco lack is consistency in play. I think it is normal that underdogs rule the league at the start of last season then their results decline as the season progresses. This can happen because the underdog team usually does not have enough squad depth to maintain consistency in play. Injuries and fatigue are what underdogs suffer from and when that time comes, they will struggle in the second half of the season.
Injuries to key players could very well result in the team being inconsistent in winning matches so that teams like Nice and Monaco are often easily left behind by PSG, who are still kings in Ligue1. But if this is caused by player fatigue, I think we need to look at the playing schedule of the two teams this season in whatever league they are in. Because if both of them only play in Ligue1, I don't think their players will get tired easily because other teams' players do too, including teams that have high consistency in Ligue1.