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.
Nice could be a great team but they are not consistent with their performance and results and it has really affected their form and position on the standing so far this season, apart from PSG, Nice happens to be another team in the french league (Ligue 1) who have got a good performance which if improved could contend strongly against PSG.
Last season Nice seemed to appear much better than they are now, but this isn't enough to consider Nice as an underdog team, Monaco could be seen in such perspective but NICE has got a performance that is considerable above average and they have finished to four on the table several times in the Ligue 1, currently they are at the second position on the table while Monaco is at the fourth position on the table even with their relatively average performance, if Nice have some level of consistency and improved performance it's very much possible they may turn out a threat to PSG soonest.