This question appears to be prompted by a reading of Aristotle's Poetics, which contains the author's most famous remarks on mimesis. The Greek word mimesis, which provides the root for our word "mime", carries connotations such as "imitation" and "representation".
In the Poetics, Aristotle plans to discuss the genres of tragedy, comedy, and epic poetry, so he has occasion to talk about how poets of these various genres "represent" or "imitate" various things. For example, each of these genres, Aristotle tells us in Section 2, represent human beings in different ways. Aristotle also discusses how the poets of different genres can use different styles of language and rhythm as a means of representation.
In Poetics 4, Aristotle tells us that people are naturally imitative, that they learn by imitating, and so they enjoy seeing and hearing the representations created by the poets. People enjoy representations because everyone learns something different from being able to identify something in a representation:
Thus the reason why men enjoy seeing a likeness is, that in contemplating it they find themselves learning or inferring, and saying perhaps, ‘Ah, that is he' (S. H. Butcher translation).
Because most of the surviving part of the Poetics deals with tragedy, most of Aristotle's remarks on mimesis are focused on this genre. Aristotle defines tragedy as "an imitation of an action that is complete, and whole, and of a certain magnitude" (S. H. Butcher translation). Furthermore, Aristotle says that tragedy should "imitate actions which excite pity and fear, this being the distinctive mark of tragic imitation." Even these sorts of representations, sad though they may be, provide the audience with some sort of pleasure.