Essentially Amiens' song in Act 2, Scene 7 satirizes the belief found in the pastoral literary genre that pastoral, or country, life is a utopia, while city life is corrupt. In fact, all of As You Like It serves to satirize certain ridiculous elements commonly found in the pastoral genre. Shakespeare uses Amiens' song to satirize pastoral beliefs by describing life in nature as both harsh yet better than the even harsher, more corrupt life found in the city, or at court.
Literary works found in the pastoral genre, a genre that dates back all the way to Ancient Greece, would actually fail to paint the harshness of nature because the literary works wanted to present pastoral life as a utopia. Shakespeare satirizes this notion by using a lot of lyrics to portray the harshness of life out in nature. For example, Amiens opens his song with the lines, "Blow, blow, thou winter wind" (II.vii.174). Amiens even describes the winter wind as having a "keen," or sharp, "tooth" and further as having a "rude" breath, which serves to describe the wind as being very biting and horrible (177, 179). All of these lyrics serve the purpose describing the harsh reality that the winter winds blowing through a snow-covered forest certainly are very harsh indeed, even fatal, and that when the winter winds blow, they certainly do not create a utopia. Hence, the description of the harshness of nature also serves to satirize the common belief found in pastoral literature.
However, Amiens is also comparing the harshness of nature with the harshness of corrupt man, and the idea that man is corrupt certainly is in line with beliefs found in pastoral literature. Amiens refers to the corruption of man by saying that the harsh winter wind isn't as "unkind as man's ingratitude" (175-76). He further argues that "friendship is feigning," meaning false, and that love is foolish (181). When Amiens refers to ingratitude, false friendship, and foolish love, he is addressing Duke Frederick's treatment of Duke Senior and saying that any love Frederick had shown his older brother Duke Senior was actually false and treacherous, as evidenced by the fact that Frederick usurped his brother, driving him into exile.
Hence, while Shakespeare is using this song to point out that pastoral life really isn't an ideal utopia, thereby satirizing the pastoral genre, he is also asserting the correctness of the idea that mankind is corrupt.