Shakespeare is criticising the use of hyperboleOver-the-top exaggeration for effect. by other poets.
In contrast he employs litotes. A litote is an understatement in which a positive statement is expressed by negating its opposite. The classic example of litotes is the phrase “not bad”.
For every conventional description he finds a way of undermining it. Instead of perfume, her breath "reeks". Her skin is not "snow" white, but "dun" coloured. Her cheeks are not full of roses.
Although there are a number of metaphorA comparison made without using 'like' or 'as', eg 'sea of troubles' and 'drowning in debt'. in the poem, they are not used to describe the speaker’s mistress. Instead, each description undermines a metaphor and she is described literally, "My mistress, when she walks, treads on the ground.”
The final line suggests that using over the top metaphors – described as "false compare" - actually "belie[s]" the object of comparison. In other words, it doesn’t do them justice.