I believe my mother had dementia when she wrote her will. Medical records say she had probable dementia. Is this contestable.

This is known as lack of testamentary capacity.  From what you have described in her medical records, there is more than enough to file a complaint and challenge the will.  It will require an expert to review the medical records, and give an opinion of whether or not someone with that level of dementia had capacity.  Capacity is not determined by a diagnosis of dementia, but whether or not the person knew what assets they had, and who their family members are.