I don't see a blanket answer here.
A beard hides the facial features - lips and surrounding areas - but I don't think it will either detract or attract attention to the area by default. It's reliant on the face and the beard.
If the goal is to hide a cleft palette, then a beard (or mustache) certainly would. But if the goal is to simply pull the eye away from the mouth, that can be done in several ways - brighter eyes and brow, less detail or an "out of focus" aspect to the lower face area, darken the lower face when compared to the upper face, angle of portrait, lighting (shadows from the nose and cheeks), etc.
In many cases a beard does attract attention, but in others it's not a competing issue.
From Duck Dynasty... Godwin's beard is nothing but an eye magnet. However, you don't focus on his lips or chin, you focus on the beard -- so it could be said that "sure, the beard detracts from his mouth" because you can't see his mouth really. However, what it does not do is detract from his lower face area. It increases the focus of that area in general.
Might be because of all the dark camo all the time
But actor Chris D'Elia's beard is purposely used, in my opinion, to hide his weak jawline and chin and pull the eye away from his actual facial features by adding the appearance of more shadows where there are none. The goal is to hide areas of his facial structure and not draw attention to them.
Ultimately, it depends upon the person and the beard. In your example using Robin Williams, the full beard pulls the eye right to it, the goatee and saved face are both pretty even to me - the chin is neither a focal point nor completely unimportant.
Most people recognize faces by shape and eyes. Beards do change the shape of a face. Drawing a beard on someone who customarily does not wear a beard will tend to make the portrait seem less accurate. Similarly, removing the beard form someone who is known to have a beard will have the same effect.