My FiL (with dementia) wouldn't change his clothes, and would only have a bath very occasionally if dh was there to,persuade him - though it still wasn't easy - and dh was away a lot for work, weeks at a time sometimes.
FiL was apt to fly into really frightening rages over the tiniest thing, so I had to tiptoe around him on eggshells. With the clothes, the only way I found was to have a clean set ready, watch like a hawk for when he went to the loo first thing, zoom in and swap. He never noticed.
With my mother (also dementia) I did occasionally 'accidentally' spill something down her, otherwise she'd be wearing the same manky jumper for weeks. I never cracked the bath/shower issue, couldn't face the dramas, but my sister, who lived much further away and so visited much less often was able to be a lot tougher. 'Come on, you NEED a shower, you smell!' (She did.)
There would be tears and dramas, but they were soon forgotten (one of the very few blessings of dementia, if there is such a thing ) and I'm sure she must have felt better afterwards.
Once she finally moved into a care home, both the clothes and the bath/shower issue were somehow managed much better. She was always clean, with clean hair, ditto all the residents, and they all had dementia.
Maybe it's easier for to accept 'persuasion' from non family, or else from staff in rather nurse-like uniforms that lend them some sort of authority.