It is not relevant, because even though rubber is non-conductive, it is definitely quite vulnerable to electric energy.
Or do you think that if you put a paddling pool over your head during a thunderstorm and got struck by lightning, you would be completely protected and escape unscathed? Concentrated electrical energy also carries a lot of thermal energy. Just for fun, the temperature of normal lightning is 5 times greater than the temperature of the sun's surface. The rubber would melt faster than butter after being hit. Not exactly ideal protection
I think that hardened amber would be ideal for MC, as it is a great solid and absolutely non-conductive element, which is technically an organic substance. Armor covered in amber plates would provide extremely effective protection against electricity. And even if MC wasn't able to create amber himself, Deryl could help him.