Well, even with the mutiny the French soldiers generally were willing to fight defensively, so the German gains were limited. If the regiments which mutinied walked away (like the Russian units in 1917), and there were more involved, then the Germans could have taken advantage. even if some mutiny in place defensively they can't be used for counterattacks. If a significant hole is opened, it is questionable how it can be plugged. At the time of the mutinies the numbers of American troops in Europe was small, and they were not fully trained and not fully equipped. The British did not have any manpower right to hand. Reinforcements from other theaters, or from the USA simply won't arrive in time.
A lot depends where the breach is. If close to Paris then Paris could fall. If along the seam between French & British forces could get to the sea and split them. Other places, not quite so disastrous.
A mutiny like this could become contagious, and as things get worse, could accelerate. Looking at the Russian collapse, and the German collapse a year later, the scenario I outlined could easily result in France throwing in the towel.