If your mech is a “Character” and not only a “Pawn”, the default character movement should handle the replication so you should not multicast it and only apply the rotation serverside.
What does your “Mech Rotation” function actually do? Does it only like “AddActorRotation” or do your mechs maybe rotate based on animation with root motion? If this is root motion based, replicating this would be a bit more complex.