If that were the sole reason, it might not be entirely accurate, as any young player with less experience can't maintain consistency, as they generally face much greater pressure when facing a much stronger team. The Club World Cup certainly looked more convincing after Chelsea's progress to the final, and this provides a glimpse into how they will prepare for next season.
From the start of the competition to the final, Chelsea generally didn't face much stronger teams, making their run to the final easier. The challenge might be against PSG, as it will be much more difficult for Chelsea to beat a team currently in top form.
Does that mean Chelsea is not improving? Chelsea actually did not face a very serious team from the beginning of the FIFA club world Cup not even in the group stage until they reach the final now but the improvement is still visible in their performance. Although, the additional players they equipped into their squad is contributing to the recent improvement. If Chelsea can approach the incoming season maintaining the current form, they can really put up a serious fight in the EPL when the new season begins.
Chelsea squad is not that bad to compete for the title in the Premier league but their major challenge has been a striker which I think is under control now since they have gotten someone who can probably replace Jackson.