Most patients get relief from symptoms of pain and stiffness after a total knee replacement. Many have improved physical function. In other words they can do more with less pain.
There may be several factors at work in your mother's case. First, it takes at least 12 months for full recovery after this type of surgery. At six months your mother still has a ways to go in terms of healing, symptoms, and function.
Second, what is her overall, general health? The presence of any other problems or conditions may delay her recovery from the total knee replacement (TKR). Finally, how does she view the results?
Many times patients' quality of life is much better after a TKR. You might not know this without asking your mother some specific questions about her own satisfaction with the results.