Masks come in various types, some of which may not always be a practical option for parties. Obviously a good number of costumes feature a mask as part of the outfit - notably superheroes/heroines (who usually use eyemasks of some sort), but in other situations such as a Masked Ball or Mardi Gras, the mask may be the centrepoint of the disguise
There are undoubtedly some excellent overhead masks on the market, and they may be especially useful when it comes to Movie Star or Character parties where one wants to capture a likeness. The problem is that almost all overhead masks can limit the ability to see, eat, drink and talk (perhaps even breathe comfortably) and, of course, as soon as one takes the mask off, the likeness is lost.
In recent years, an alternative has been the cardboard 'photo masks' of celebs and personalities. These still cover the face (unless you wear them on the back of your head and confuse everybody) but they are easier to work with. Antrix has an ever-changing collection of these for sale.