Do some people with moderate to late stage dementia sleep most of the time?

Confusia - your mom sounds a lot like mine, she was given sinemet for Parkinson's but I always suspected it was more likely Parkinsonism brought about by multiple TIA's and perhaps a small stoke.

No one has ever told us what type of dementia she has.
In 2017 she had an MRI which showed severe ischaemic changes. She has been diagnosed with Parkinson’s but doesn’t have tremors . She has slowness of movement and very unsteady gait. Her tone of voice is very soft
Her short term and long term memory are okay.
we haven’t been able to see neurologist because of COVID and telephone appointments are not helpful because my mom doesn’t partake. When I ask GP anything he says it’s the condition.

My mom slept a lot too, at least 18 hours a day while she was at home and once she was in the nursing home she was seldom awake except for meals and when she was receiving care.

Sleeping a lot is a sign of decline, one that Hospice uses when recertifying a patient.
My Husband I think started sleeping 12 hours a day then over the course of a few years he slept more until the last 6 months he slept between 20 and 22 hours. Awake briefly when I got him up, got his shower then breakfast. then he would sleep until lunch, sometimes through lunch until dinner then he would sleep through the night. The times he was awake during the day was when I changed him, sometimes when I changed his position.
(There is a pamphlet called Crossing the Creek that has a great explanation as to the "why" so much sleep towards the end of life. It is not a scientific reason but maybe a philosophical one. )

My mom is 90, not officially diagnosed with dementia but has no short term memory at all, and poor decision making sometimes. She sleeps in the chair as she is bored. Having 2 different brain disorders makes it hard to know what will help, in terms of meds or activities. A trip to the neurologist who is treating her might be a good idea - to get an idea of her stage of decline and what are reasonable expectations.
Also, get in touch with the Alzheimer's Association. Their stuff is very helpful for anyone dealing with dementia, of any type. There is also support at your local area on aging. www.eldercare.gov
Good luck.

My husband who had vascular dementia in the last 6 months of his life slept about 18-20 hours a day.
As you are already aware, vascular dementia is the most aggressive of all the dementias, with a life expectancy of just 5 years(my husband died in about 3)so she may just be nearing the end of her life.
Perhaps it's time to get hospice involved, as they can be very helpful with letting you know what is going on, and they are covered 100% under your moms Medicare. It's never too soon to get them involved. Best wishes.

My mother slept about 16-18 hours a day when she was still at home. I think much of it was just boredom. Once we moved her to a memory care, she was kept more engaged by the staff and didn't take naps like that until the last seven months of her life when she really started going downhill.