So Miraak's mask can be either Light or Heavy Armor, but what's the purpose? According to the Elder Scrolls wikia, there's no statistical difference.

The only advantageous thing I can imagine is that if it's Light Armor, you can still pair it with Miraak's Gloves and Boots for better armor rating with the Light Armor skill. But even then those only exist to improve Miraak's Robes, which are not classified as armor, rendering this advantage totally pointless.

Perks most likely