Assalaamu alaykum. My mother told me to smile more and that I looked to serious after she saw me. I do not like to force myself to smile or to be fake. However, I know that one should listen to their parents. To what extent does one has to listen to their parents?

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, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Whatever is within your ability to do that does not exceed the common limits of smiling and cheerfulness where appropriate, obey your mother therein because it is a lawful deed that involves no harm to you and it pleases her, which is part of your dutifulness towards her. Abu Tharr, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, "Smiling at your brother's face is an act of charity." [At-Tirmithi]

Please refer to fatwa 131695 about the limits of the children's obedience to the parents.

Allah knows best.