PSG finally wrapped up the title on Sunday after second-placed Lille failed to beat Toulouse, having not won any of their previous three games when the opportunity to confirm a sixth French crown in seven seasons was there.
The Qatari-owned club can still make it a domestic double, with the French Cup final to come against Rennes next weekend, yet this will not be remembered as a vintage year.
While Neymar once again went missing due to injury at the crucial point, Edinson Cavani has struggled with fitness too, and coach Thomas Tuchel has regularly lashed out at the lack of depth in his squad.
The 5-1 capitulation at Lille a week ago was the worst league result for PSG since Qatar Sports Investments bought the club in 2011, and the first time they had let in five in a league game since 2000.
A record-breaking 14-match winning run with which they started the season now seems like a long time ago, the latest Champions League failure clouding Tuchel's first campaign in charge.
Mbappé, at least, has been a constant, revelling in his status as a World Cup winner and hardly ever being rested. He only turned 20 in December, but has now won three Ligue 1 titles in a row.
Mbappé has 36 goals in 40 games this season, with 30 in the league. And, frighteningly, his team-mate Daniel Alves told RMC recently that “he doesn't realise how good he is, he can go much further”.
He is also not used to losing, and may have upset some of his teammates with his assessment of last week's defeat at Lille. “We played like beginners,” he said.
To be fair, they have usually been exceptional domestically. However, the problem is that as long as they keep failing in the Champions League, domestic success for a club backed by a Gulf state will continue to ring hollow.
The title has effectively been in the bag for some time, but their season has been winding down ever since their exit against Manchester United in the last 16.
That, combined with being knocked out in the League Cup quarter-finals, means Tuchel in his first season has actually done worse than Unai Emery, his unloved predecessor.
Paris Saint-Germain coach Thomas Tuchel has actually done worse than his predecessor, Unai Emery
However, according to sports daily L'Equipe, Tuchel has agreed to extend his contract to 2021. Thoughts have already turned to the future, with new signings needed at the Parc des Princes.
The costly Neymar experiment cannot be said to have succeeded until he stays fit for the games that matter, and there will always be speculation about his future.
“We have a contract, we're not even halfway through that contract,” Neymar's father pointed out to RMC Sport.
Time for a clear-out?
Mbappé is the one man they really cannot afford to lose, but is it time for a clear-out elsewhere?
Some of the excitement of the early part of the decade has gone, replaced with a staleness. The days of Thiago Silva, Edinson Cavani and Marco Verratti are maybe ending.
Youri Djorkaeff, a World Cup winner for France and a PSG star in the 1990s, offers a frank assessment, suggesting real change is needed higher up.
“If you're not great in every compartment, from the bus driver to the girls who clean the shirts, you will go nowhere,” he told Ligue 1 Podcast, 'Le Beau Jeu'.
“Paris Saint-Germain, after many years without success, have to rebuild everything, restart from scratch, because the foundations are not good. You cannot expect to one day win the Champions League without these foundations.”
The Qatar project needs a breath of fresh air, and all eyes will be on president Nasser al-Khelaifi and sporting director Antero Henrique.
UEFA's Financial Fair Play rules remain a problem, though, with PSG said to have a hole of around 100 million euros and the need to present a balanced budget by the end of June. The next few months will be interesting.
By Andy Scott/AFP