Is what a mother does, in insulting and hitting her 21-year-old daughter for the most trivial reasons, and describing her with humiliating terms in front of others, acceptable in Islam? And is the daughter considered disobedient to her mother if she rejects this treatment, and will Allah be angry with her?

If the mother's situation is as you described, it is a great trial that requires patience, which leads to a good outcome, elevated ranks, and expiation of sins. The mother's right to dutifulness and kindness remains, even if she acts poorly. Advice should be offered to her gently and kindly, preferably by someone who holds a respectable position in her eyes. You must be patient and beware of disobedience, and avoid everything that provokes the mother. Mere advice or refusing a beating is not disobedience unless it exceeds the bounds of proper conduct. I recommend supplicating for her well-being, that Allah grants her forbearance and deliberation, and removes from her quick anger, foul language, and rash behavior. Strive to occupy your time with what benefits you in your religion and worldly life, associate with righteous women, and seek help from Allah.