"Your mother's, like, a total hypocrite; she's like a C-cup."

Bra sizes are determined by the circumference of the chest and the circumference around the chest over the center of the breast mass. The former determines the "band size," and the difference between the former and latter determines the "cup" size. A 32C bra means that a woman's chest rates at 32 inches and a C-proportion additional depth to accommodate the breast.
I a woman is a C-cup, her breasts are full enough to be prominent, although they are not huge.
So Esther Horan is saying that if breasts are bad, then Margaret is a hypocrite because it's obvious she has breasts.

Joshua 1:9 ... unashamed.