This is a misunderstanding.
圣母 in Chinese internet slang means “A person with the purest virtues and highest sense of honor, a paragon of mankind”, it’s an absolute praise.
The slang you want to ask which means a “hypocrite” is 圣母婊. Or a “Virgin Mary Bitch”.
The expression is an abbreviation of “操着圣母的心，做着婊子的事” (“Have a heart of Virgin Mary, do what a bitch does”). It’s a high contrast. So it really does make sense because if we understand this abbreviation, we understand where the “Hypocrite” part comes from.
Some examples:
A “Virgin Mary” in Chinese slang:
You are shocked by the state of poverty in a remote village of Gansu Province. You volunteer to teach math, English, and literature in those villages for one whole year. You live in stone age huts, drink well water, eat pumpkins, take a bath monthly, or quarterly.
A “Virgin Mary Bitch” in Chinese slang:
You are shocked by the state of proverty in remote village of Gansu Province. You blame others for not volunteering to teach math, English, and literature in those villages for one whole year, others for not willing to live in stone age huts, drink well water, eat pumpkins, take bath monthly, or quarterly.
“Virgin Mary” =>
You sit on a crowded bus, see an old man get on, standing. You give up your seat to him.
“Virgin Mary Bitch” =>
You sit on a crowded bus, see an old man get on, standing. You start to rant about how everyone else is selfish not willing to give up their seat, while sitting.