The metal in a Mark V helmet is actually a lot thinner than it looks, probably around 1.5mm - 2mm thick. Making the helmet shell thinner would make it less resistant to impact damage, and could affect its' structural integrity by the time the ports and all the other necessary fittings are put in place.
A standard Mark V helmet weights 54lb/24kg, which makes it one of the heavier standard dress helmets ever made. Making the helmet out of thinner metal wouldn't actually make the diver's gear any lighter, because any weight lost from the helmet would need to be added back somewhere else to compensate for the buoyancy of the helmet.
That said, full Mark V dress is still particularly heavy as standard dress goes, for a couple of reasons. Because the Mark V helmet is relatively large (especially compared to Chinese and Japanese helmets) it needs more weight to sink it. It also makes the diver's visibility relatively poor, because the ports are further away from the diver's face, reducing the angle of view.
The whole configuration of Mark V dress (including the weight belt and shoes) was also designed to enable a diver to remain firmly anchored in place in a strong current. I know some people who have done buoyancy tests with Mark V gear, and established that the Mark V weight belt can safely be reduced by about 39lb/17kg and still be enough to get a diver under water in calm conditions. This reduces the total weight of the gear from 190lb/86kg to 151lb/69kg, which when you compare this to a tech diver with a full set of twins and a stage cylinder, isn't that much different.
