They're not exhausted. They played like this much of the Toronto series. They just play uninspired hockey.
It can be both. And they were clearly fatigued in the back half of the Toronto series too. The physical malaise set in early. The mental aspects and the general character of the team and the way they go about things sit on top of that, for sure, and are arguably more important. But the fitness and condition of the players is absolutely a factor too, IMO.