1. Mascarille's coarseness often shows through his polished surface - a fact that Moliere uses to great effect to show us that truly fine manners can arise only from inner goodness. Because Mascarille is a scoundrel (or close enough to one) his manners must sometimes slip to remind us not to be taken in by his presentation. He is himself aware of the fact that he is sometimes polished and sometimes coarse, as he notes in this description of himself in I, ii:
A truce to these compliments; when people have need of us poor servants, we are darlings, and incomparable creatures; but at other times, at the least fit of anger, we are scoundrels, and ought to be soundly thrashed
2. Neither of the two "precieuses" sees through the fraudulent front that Mascarille puts up for the very sound reason that it is very much not to their advantage even to consider that people might not be what they seem in terms of their public presentation of themselves. If they were to entertain this notion about Mascarille, they might well also have to entertain it about themselves, and this is something that they do not wish to do.
3. Mascarille entertains his feminine audience by reading from his store of writing - to great comical effect. This effect arises both from the wordplay inherent in his reading (some of which does not translate into English but which is wickedly clever in French) and in part from the disjuncture of the style of the rest of the play - with its quick, sardonic commentary - and Mascarille's attempts to raise the tone to something grander and quite a bit more long-winded.
4. In satirizing the work of Mascarille as writer Moliere was in fact satirizing an entire school of writers who were enchanted with their own eloquence. Mascarille mocks this style in II, xiv
To praise you as you deserve, I lack eloquence; and feel unequal to the task. Yes, sufficiently to commend this lofty effort, this fine stratagem of war achieved befor...