It depends. Using a helmet on someone’s head? Yes.
In this case, it would be an easy self-defense acquittal. On a football field, it was inappropriate and deserved a suspension and fine. But in court, if someone attacks me and I have something I can use as a club in my hand, I expect to be allowed to use it.
Note - I don’t agree with the folks who say Rudolph started it. Garrett dragged him down well after the initial hit and then rolled on top of him and got in his face. That was when Rudolph tried to grab his helmet.