In football there are sometimes that loses are bound to happen and despite the fact that PSG lost that game go Chelsea doesn't necessarily mean that they will be lose in their game in league 1 because with the quality and standard of that team I don't think any other team can actually challenge them for the league 1 title because they will be winners at the end of the season again and besides lille and Monaco aren't actually big team to give PSG headache.
PSG clearly still reigns supreme in Ligue 1, regardless of what happened to them outside of Ligue 1. PSG's loss in the Club World Cup final against Chelsea is quite commonplace, as Chelsea have been in good form lately, so it's not fair to consider them a weak team. So, what happened to PSG unexpectedly is something they must accept, as they were simply unlucky in that final. However, for next season's Ligue 1, I don't think any other team will be able to stop them from winning the title again and again.
The defeat was quite surprising but it does not mean that with this defeat PSG is considered a weak club they are still strong even in League 1 no one can beat them moreover PSG must be quite satisfied this season they have won the title they dreamed of, namely the Champions League and failing to win the CWC title will not make PSG too disappointed at least PSG is also quite satisfied to be the 2nd winner, next season PSG is not only considered the strongest player in League 1 but in the Champions League PSG will be more feared and will compete again in the hunt for the UCL title with top teams.