An individual who wears a figurative mask would appear to be something entirely different on the outside from what the individual is on the inside. One of those individuals would be Malvolio.
As we see Maria describing in Act 2, Scene 3, Malvolio comes across as being a flatterer and so dignified that "all that look on him love him" (II.iii.139). He is further described as being very proud and even as a "kind of Puritan," meaning that he is excessively virtuous and strict, just like the Puritans (129). He is so proud and dignified that he frequently insults other characters, such as when he insulted Feste's abilities as a court jester in Act 1. However, though on the surface he appears to be virtuous, upright, dignified, and well admired, Maria, Feste and the other characters soon prove that on the inside he is really nothing more than a gullible fool who quickly looses all sense of dignity the moment his foolishness is exposed. Maria proves he is a gullible fool by convincing him through a letter that Olivia is in love with him and wants to elevate him to her station. Feste proves him to be a fool when he manages to deceive Malvolio into believing that Feste is the curate who has come to diagnose him as insane.
Orsino is bound by his own mask of love. The mask is a distorted sense of love and is fed by boredom, lack of physical love, and excessive imagination. His "love" for Olivia is superficial unlike Viola. Had Viola kept her feminine costume, she would never have been privy to Orsino's intimate feelings; but, as Cesario, and as a male, she has a seemingly physical connection with Orsino.
I would also consider Olivia. She becomes aggressive in her pursuit of Cesario. The love that she has for Cesario's "male exterior" causes Olivia to transform from the socially acceptable "weak" female and into the aggressive male. At some point, Olivia is not in love with a disguise, but in love with the actual person (Viola) that wears the disguise. Once the "masks" had been removed, and the falsities had been acknowledged they can all have happiness.