One thing I found with my own mother, who had vascular dementia, was not to ask if she wanted to do anything. Her answer was always no. I learned to just do whatever needed to be done and she would go along with it. I don't know the situation with the living arrangements with you and your mom. Is she still at home? Are you there when she gets up in the morning? If so, go ahead and get a pan of warm soapy water ready and give her a bedside bath before she gets out of the bed. Don't tell her you are doing it, just do it. Have clean clothes ready to put on. It's possible if you don't make it an issue, you may get better results.
Before my 97-yr old Auntie's dementia progressed her caregiver used to tell her they were going someplace and so she needed to get ready. This worked for a while. I also have seen that the "don't ask, just do" approach does work.
Do you have a chair for her to sit on. The fear of falling is one reason. Do you have safety bars? A non slip mat? A warm enough bathroom?
If none of these things are an issue just do a sponge bath. There are no rinse wipes that are available or even a warm wash cloth with soap.