The "low energy" came from the club falling behind due to the turnovers. They drove down the field and answered Dallas' field goal and tied things up at 3-3. It's not like it was terrible. Then, the defense adjusted and played well, but the offense fell off a cliff after Jones' two turnovers put them behind again against a pretty good Dallas offense. That put them in a hole that the defense spent the rest of the afternoon trying to give them a chance to get back in it. That obviously never happened.
To your point, Belichick will be fired when it's clear he's lost the locker room or games like this become more of the norm. I don't think we're there yet.
Not saying I'm not concerned we won't see that, especially if their two best defenders are gone and they fall apart, but I don't think energy/effort was the issue today. They ended up down 28-3 not due to their own (the defense's) volition, but because of two really disastrous decisions by Jones. Hard to be happy and upbeat about that.
Either way, I don't think Belichick's the problem from this standpoint. Although, as I said earlier, I do feel like after watching Miami and Buffalo today, that the fact there's not one truly explosive player with a quarterback on a rookie deal is incredibly frustrating.