Touchstone, as a Shakespearean fool, is a fascinating character who seems to perform many functions and purposes in this play. On the one hand, it is possible to argue that he performs the role of using his position as "fool" to actually comment on what is happening around him and other characters in a way that reveals the truth rather than nonsense. This is something that can be seen in the way that he functions as a "touchstone," a character that exposes the value of other people around him. His conversation with Corin in Act III scene 2 is a perfect example of this, as Touchstone shows the beauty and simplicity of Corin's way of life.
Another view of this character is the way that his opinions about love and marriage, shown through his hilarious relationship with Audrey in Act III scene 3, offer a much more brutal and realistic vision of the politics of love when compared with the relationships of Orlando and Rosalind and Silvius and Phoebe. Note what he says to Audrey when she acknowledges she is "foul," by which in her simplicity, she means dirty as a shepherdess:
Well, praised be the gods for thy foulness. Sluttishness may come hereafter.
Touchstone here acknowledges in a play on words that his wife is "foul," but then also states that Audrey will likely cuckold him or be unfaithful to him later on in their relationship. For Touchstone, this marriage is about satisfying his lusts and nothing more. Therefore, Touchstone can be viewed as one way in which Shakespeare challenges the pastoral ideal of a courtly lover going to the countryside and marrying an unrefined country girl. Audrey certainly is unrefined, and Shakespeare parodies the pastoral relationship through the coupling of Audrey and Touchstone as part of his bigger theme of exploring and questioning pastoral literature.