What should I answer when my mother shouts that someone has brainwashed me and that I look like a monkey while performing prayers?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

First of all, you should know that giving help to one's parents is their due even though they may be disbelievers. Allah Says regarding the disbelieving parents (what means): {But if they (both) strive with you to make you join in worship with Me others that of which you have no knowledge, then obey them not; but behave with them in the world kindly and follow the path of him who turned to Me in repentance and in obedience.} [Quran 31:15]

Allah did not drop the right of parents to benefaction, though some of them spare no effort in their scope to eliminate His right to be worshipped Alone with no associates. The right of the father is great. But the right of Allah is much Greater. So, the father should not be obeyed, if he orders the child to do something in disobedience to Allah. No one should be obeyed in orders that are against Allah's Orders. An exemplary method in your treatment towards your mother, if she went back to what you have mentioned in your query, is to ask Allah to guide her to the right path. Please listen to her words till she finishes, without caring for what she says. Do not cause her to become furious. Then go back to your duty and do not respond to her and when her anguish vanishes, speak to her kindly help her in her domestic affairs, and accomplish her needs, and always show good treatment to her. Tolerate her infliction with good will and make your ardent invocations for her guidance.

Allah Knows best.