My wife's grandmother has suddenly gone extremely mean and inappropriate.?

If it's sudden, then I would say it's not dementia, or at least not just dementia. Dementia is usually a gradual process, if she's had a very sudden change in demeanor then I would suggest getting her to the doctor and having tests run, in the testing make sure the doctor knows that her behavior has changed very suddenly and is completely out of character, focus more on the inappropriate behavior rather than the "shingles", this could be signs of a larger problem.