It's true that they've had some clutch problems over the past two years. Still, I'm not sure Max was ever renowned for his starts the way Alonso is (was?) for instance.
It's the car. Red Bull has commented in the past about trying to reduce wheel spin at the start of races by rolling on the power more slowly. They're better than they have been, but clearly not as good as McLaren.
They've had trouble with the clutch for the past two years, but even so I wouldn't hold the starts specifically as Max's best attribute. But that is more because there are other parts of driving where he is just unbelievable.