anteolsson: I am not so sure of how people's teeth looked like back then. It is certain, though, that they did not have the same problems that we have now as their sugar intake was not in comparison to modern times.
I saw a archaeological documentary where they found a 600 year old sunken ship, somewhere between Finland and Sweden. The people (read skeletons) they found on the ship had immaculate teeth and the people were seafarers.
It is easy to think that humans had worse teeth in the past as our technological advances has given us super tools to enhance and mend our teeth. Sugar is, however, the biggest factor to why we have to mend our teeth, and sugar was not prevalent in those days.
Geralt could of course still get brown teeth even if they are healthy in another sense. So, if it were possible to keep your teeth not-so-brown then this mod might not be such a bad idea, as long as it is put in the right context of course, ie not compared to the look of modern teeth.
it is quite funny though, reading this thread.
I think dairy would have been a bigger problem than sugar. Talk about bacteria magnets.