Christina was a Brat!
Joan's abusive behavior was no excuse, but let's examine some of the things that triggered her tirades with her daughter--
1) Christina, especially baby Christina, was not a very loving child. If you listen to John Waters' commentary on the DVD, he even states that this blonde child was albino, pale and very "distant". Joan wanted someone to love her back, and Christina was a very cold soul. She also was a Gemini- and if you understand zodiacs and energies, Gemini's are very mutable and unstable beings (think Donald Trump, Kanye West, Marilyn Monroe). They are masculine, outgoing creatures but they don't like to be tied down. Crawford was an Aries. And although Aries are competitive and also strongly manly in their energy, they also demand and need respect and love in order to give it back. It's hard to get that from a Gemini. In fact- of all the zodiacs, Gemini along with Sagittarius are the least-emotional givers.
2) Christina not eating her meat. AFTER being told by Carol Ann that her mother was going through a difficult time by taking the screentest, Christina acts like a spoiled bytch at the table. Refusing to eat the lunch her mother provided her, she instead snaps off. "I've had my vitamins today- pills!" Back talking will get you slapped. She was ungrateful and rude, for no reason.
3) Christina was a enabler. She purposely barged in on her mother when Joan was trying to have company sexually. No wonder she was sent away to school. Why was she all in her mom's business like that? Girl go back to your room. I didn't blame Joan one bit for sending her off. Christina's behavior was borderline psychotic. What was her obsession with always glaring at her mother with those weird, manly eyes and darting back and forth for no particular reason?
4) Christina mocked her mother. From the mirror scene, to talking to the dolls- Christina knew how to get under Joan's skin- and would manipulate answers as to why she was doing it. "I was play acting." Yeah right. Putting on your mother's expensive tanning lotion? You knew you were being invasive of her things.
5) Christina making LOUD noises the day Joan was meeting with the mayor. "You promised Christina, you promised." But being a Gemini, they are forgetful and careless. Christina didn't seem to mind making all that noise and her mother was of course, fired hours later. It seemed to Christina not make any difference. She did as she wanted, no matter what she was told before.
As she gets older, I suppose she matured. She was innocent in the scene with the guy who made out with her. She was not expelled from school like Joan lies to Barbra about. And she certainly didn't deserve the physical trauma her mother caused her - like wrestling her to the ground, and then sending her off to stay with that nun. But in the earlier years, Christina was not a very loving child. She seemed to like to toy with Joan, and being vulnerable- Joan fought back the only way she knew how. There's always two sides.