My mother (86) is constantly rubbing her left temple and when she is eating. Is this part of dementia?

My father in-law does a lot of repetitive motion actions such as rocking back in forth in a chair or strumming his fingers and rubbing his face also.
Just from my observation alone it seems to comfort him in some way and so I have not discouraged it as it is not hurting him in any way.

Possibly your mother is doing the same.

Apparently obsessive rubbing of the face is not uncommon in Alzheimer's. It would be good to talk to her doctor, though and see if she should be examined to rule out pain or other causes. Does she only do this when she eats?