Elisha, the implication is such:
1. “The only wisdom of a woman is in sewing.”
2. Oomis (“Bubby”?) “cannot sew to save her life.”
Therefore, since she doesn’t have that particular wisdom, which according to R’ Elazar is the only wisdom that exists for women, she would be classified as “unwise” or “stupid.” (I too fall into this category. I suspect many other women do too.)