I'm afraid the only way we ever overcame the not washing syndrome was to have carers come in and do it with her. They came every weekday at 8 and got her showered twice a week and washed on the other days. That way, when mum went to day care or one of her groups, I did at least know she smelled OK and was wearing clean clothes. If the budget doesn't allow this, try catching her when she goes to the loo first thing in the morning; be prepared with clean clothes on the radiator and an assertive, "Well, now that we are here and your clothes are warming nicely, let's get you in the shower."
Someone pointed out that this might cause mum not to go to the loo (!) but, fortunately, the dementia and resultant lack of recall about what had happened the previous day overcame that problem.
I think it comes down to having to supervise virtually all their personal needs care in the end, be it showering or just getting out their clean clothes and removing the dirty ones. And you develop the skin of a rhino of course! You mum does not mean what she says, she is reacting to being told what she should do and the dementia means she has lost the ability to reason.
I was various people to my mum over the years from her best friend to the lady who booked the church hall for the keep fit classes. She claimed she hardly ever saw her daughter as she worked in London for a publishers (that was the 1970s). So I became her best friend, or the lady who booked the church hall, or whatever. She of course never forgot her son, who we never saw from one year to the next, but he, apparently, came to see her every day.