Is there no penalty for using heavy armor on mages?

[Nemerlight]

Since heavy armor looks the coolest

[barzai]

The only heavy armor mages can wear is crafted armor made from Silverite. Otherwise they cannot wear heavy armor at all.

Silverite is not easy to obtain, the only regions it is readily available in--that is, not as a tiny supply from a sack or similar--are the Arbor Wilds, which are only accessible in the main story quest What Pride Hath Wrought, and the Emprise du Lion, which is pretty high-level.

Oh, and there is a sorta-kinda War Table quest in the Deep Roads DLC that provides Silverite.

[Mephisto-Luzifer]

You could use a mod that removes class restrictions for armor.