Is it normal that my mother who just had a knee replacement is complaining about pain on her other knee?

Yes, it's normal. Your mom probably favored her good knee prior to the surgery which put more wear and tear on it.

Unfortunately most people expect miracles from knee and hip replacements.
One has to consider the age and general health of the joint before surgery and expect to suffer some pain despite rehab and a normal surgery.
Surgeries can go wrong but usually the pain is caused by an unconscious tendency to put more weight on the good side of the body and therefore avoid anticipated pain from the side that has been operated on.
Pain is natures way of protecting injured areas of our body. Masking the pain is often the wrong solution.

My mom had total knee replacement in one knee, and also complains of pain in the other knee. She was told by the doctor that it is common for the 'good' knee to hurt because it is supporting while the replacement knee rehabilitates. However, she was told that both knees were "bone-on-bone" and the other knee would eventually need replaced. Our hopes were that the one replacement would lessen her pain for a while, but it has been 7 months and her pain in the new knee is somewhat better than before, but still painful. The other knee is very, very painful and she cannot walk more than 20-30 feet at most. She takes acetaminophen and ibuprofen but still complains of pain when she moves about. Since sitting is less painful, she sits most of the day. She is also obese, and does not eat healthy, which I know is lending to her pain. The doctor says she shouldn't have pain in the new knee, but I don't think he realizes how little she uses it. It's very disappointing that she went through so much to get the knee pain free, but it ends up like this. She is 82.