Sure grip was an issue. But I think its pretty clear its easier to deflect lightning with your lightsaber than without.
If we look here at 2:20:
Mace has to move back to catch Palpatines initial lightning blast on his Saber.
Then if we look here at 3:31:
Yoda is literally on the edge of the pod, so had no room to move back. So He was just not in the best position to keep a good grip on his lightsaber against Sidious full blast.