My mother has dementia and I guess she has lost her sense of smell. What do I do?

Diamond, I feel for the cat as they do not like a dirty cat litter and will start going elsewhere in the house, or use the bathtub. Does the cat have more than one cat litter box? If not, get a second one to help with the issue.

As for cleaning the house. I wouldn't dare help clean my Mom's house, she would be offended. She even said "no" to having a cleaning crew come in... my gosh, if someone offered me a cleaning crew I would be at the door welcoming everyone in :) My Mom was in denial that the house wasn't her usual white glove clean.

I wonder if you could use the excuse "Mom time for Spring Cleaning, I am here to help".

With the dementia, she probably doesn't realize that the litter box needs changing. I'm not so sure that it's about smell, but, they just forget or they lose the initiative to do things. It's really not fair to expect her to care for a pet. I'd take over the duties to care for the cat and/or figure out what to do with the cat. She may also overfeed the cat. Unless, it's the early stages, people with dementia really need supervision all the time. Do you live with her?

She likely has no awareness of the smell or what might need cleaning. I'd just read a lot about how the dementia causes such changes and how they really can't be expected to function the way they used to.