The best paraphrase of line 12 is "like everyone,she walks on the ground"
Throughout the sonnet the speaker is portraying his beloved as someone ordinary. He highlights the common characteristics and beauty of his lover using the conventional comparisons of the Shakespeare's days for love poetry but in a negative way.
The love poetry of the those days was mainly based on metaphors comparing nature with the beauty of the loved women. In this sonnet Shakespeare reversed those conventions.
In line 12 and in the previous one the speaker states that he has never seen a goddess walking but he's sure that her mistress walks like everyone else.
