I feel for you justanovice as my late MIL was diagnosed with two types of dementia. Also fiercely independent, we had battles to try and get her to wash, change, eat or take medication. Her eldest son and his wife got the worst of it. She accused them of being paranoid about germs, "clean clothes aren't necessary, I'm clean!, my health is more important!" Not seeing the connection.
Other DIL would offer to take her out for shopping or coffee, "But only if you change your clothes," which sometimes worked.
It only needs a young great-grandchild to say, "You smell, gan-gan!" It's difficult, dementia changes or exaggerates their personalities.
Professional carers seem to have the knack, "I/friends etc will be happy if you change your clothes," I sympathise - it doesn't seem to make any difference if you are polite or rude to them - they think that they are right and you are wrong, how dare you, etc. No easy answers.