My mother is eating less, and has become frail and more unfocused. What can we do?

It sounds like her disease is progressing and she is declining. As always check with her doctor to find out what (s)he says. It may be time for hospice to get involved to make sure she is kept comfortable. I suspect all you can do a this stage is to love and care for her, see that her needs are met. I know it is difficult to see a parent's abilities diminish like this. ((((((hugs))))

This is a typical decline with Dementia.
You can contact a Hospice and ask for an evaluation, I am sure she would be eligible. Hospice staff will help with supplies and equipment but most of all would be there to support you and let you know what the next stages are. The primary goal will be to keep mom comfortable.
Do expect that at some point she will stop eating and or drinking all together.
PLEASE resist the urge to have a feeding tube put in.
Nearing the end of life the body does not utilize food as we do. The energy that food provides sill go to keeping major organs going. The heart, brain and excess food/calories can be more of a problem.
Using a swab to keep the mouth moist will help somewhat.
Breathing will change and there will be noises from fluids that she will be unable to swallow. Turn her on her side to allow them to drain. Or use a swab to absorb some. But do not put a swab back into the throat as that can cause a gag reflex. The noises sound bad but are not painful to her.
((hugs)) and prayers.

It's only natural to notice decline but trying to find the things she still can do & enjoy can be rewarding. The Teepa Snow gems model may be useful, especially Ruby stage.

You have some very helpful responses here, especially from Grandma 1954. My wife died a few weeks ago at the age of 84 after some 12 years with Alzheimer's. The illness does progress. We fed her for the last two and half years, but she remained friendly and communicative despite being incontinent and confined to bed.

It is important not to give her too much food; and I agree with Grandma 1954 that it is best not to use feeding tubes. If she is in pain, small doses of morphine will relieve the pain and help her to decline and die peacefully, but that could be many (or few) months away.

Try to get help help at home. There is a franchise called "Home Instead Senior Care" that will send in caregivers for whom you pay a reasonable charge per hour. They are many local branches in both the USA and the United Kingdom. You could see if they can help. Also, hospice care might be necessary later, but it is difficult to know with dementia how long the process of slowly dying will last.

Take care, give care is somewhat a guideline for living at this difficult time when COVID is still dangerous.

Love and prayers

Thanks for the responses. Mom's feeling a bit stronger today, this also because she's started eating a bit. Still, we are feeding her.