This mask?

Frankly, it ain't frankish. This is likely a choice made by the costume designer or director to have some kind of character symbolism (hiding true self/true feelings). You have "battle masks" in England, and ceremonial/mummer masks for various holidays, but you don't have this kind of thing for the upper classes.

The closest practice to this seems to come much later, well into the renaissance/baroque period. There, you see many upper class people wearing masks when they go out and about - both to protect their complexions, and to conceal their movements. Even then, they were fairly stylized - the most popular type being french & english vizards, a black, featureless mask, and the Italian bauta with its prominent beaky-ness.

You have to remember that the costume designer's main goals are to serve the characters and the story - historical accuracy is not the highest priority, and often period-accurate clothing is very, very strange and off putting to contemporary audiences. Many of the costumes in Vikings are not historically accurate (for example, this is pure fantasy) so the odd mask on Charles is no different in that regard.

Edit: Found a quote from the costume designer, regarding historicity -

Bergin admits that she does try to make historical clothing accessible to our time. "The era when every detail had to be historically accurate is gone. You must try to learn what is correct and give a modern sensibility to it. The more I work, the more I believe that"

I know that the character in the show is Charlemagne's grandson, but, ever since I first read of Charlemagne, I remember seeing a golden bust, or possibly a mask of Charlemagne, that seems very similar to the one in the show. I believe that that mask might have inspired this one.

I'll try to fond the picture and post it in the edit.

Edit: Found it! It seems to be a bust rather than a mask, but a representation of Charlemagne nonetheless. Here it is : http://www.wga.hu/art/zzdeco/1gold/14c/06g_1300.jpg