Hello!
The answer is: the sentence is grammatically correct.
Why?
Mi mamá se sienta en el sofá / My mom sits on the couch/sofa.
Since it's describing an action that a person is doing at the moment, it's grammatically correct.
Mi mamá/ My mom = subject
Se sienta / Sits = action (present)
en el sofá / on the couch/sofa = complement
Have a nice day!