Is it common or expected for a person (58 yo) to continue to...
Continued deterioration after a stroke is not common, and it sounds like she needs further evaluation.
There are situations where a patient can become more stiff as time goes by, as the spinal cord takes over the function that the brain used to provide, and causes tightening of the muscles of the affected limbs. However, a deterioration in mental status is not normal, and makes me think something else is going on.
I would recommend an evaluation - with repeat imaging of the brain (preferably with MRI), blood checks (specifically evaluating for thyroid level, vitamin B12 level, and signs of infection), and possibly an electroencephalogram to look for signs of subtle seizures that can develop after a stroke. An additional thought is that she is suffering from severe depression and has not been exercising or moving the muscles enough.
The bottom line here is that deterioration this far after a stroke would not be expected, and should be evaluated by a Neurologist.