The best and most simplistic answer to this question is: honestly.  Shakespeare is honest about his love, and describes her as an average person.  He says here eyes are "not like the sun" - meaning they are not overly bright.  He says that her is black - "black wires grow on her head".  He says that her breasts "are dun", meaning that they are not pure white but a more faded and average skin tone.  In Shakespeare time, pale white skin was the most attractive; browner skin suggested a hard-working or outdoor life.

Going on, Shakespeare explains that her breath is not like perfume, and her voice is not like music.  Again, he is being honest.  He is saying that she is not perfect.  Shakespeare does this in reaction to so many love poets that over-glorify their lady loves.  The metaphors used by the other poets suggest that the woman are like goddesses and are meant to be worshipped.  Shakespeare refuses to make that comparison here.  He actually refers specifically to that:

I grant I never saw a goddess go

My mistress when she walks treads on the ground.

However, his argument is that his love is even more pure than the love of all those flowery poets, because he is able to admit the flaws and limitations of his mistress and love her all the same.

And yet, by heaven, I think my love as rare

As any she belied with false compare.

He is implying that other poet's compare falsely, and so their love is not as rare or as pure.  His - being based in reality - is the true love.