OP, my mum was a hoarder. She showed signs of it when I was a child, but the older she got, the worse it got.
When I was a child there would be a huge clean up once a year and she maintained that for months, then everything would build up again.
She was born in 1930, right at the very end of the Great Depression. She was a young teen for the Second World War, she literally grew up with the idea of never throwing out anything that could be useful (even if to everyone else, these items looked like junk). She never kept actual rubbish thank goodness. She still washed her dishes and put out the rubbish, still vacuumed what could be seen of her floors and cleaned the bathroom. But, she didn't use the bath, so that was piled past my height with stuff. She no longer slept in any of her three bedrooms, because they were full of stuff (she slept in her lounge room).
When I told her I was pregnant with my first child she couldn't have been happier. Vowed she was going to clean her place and make a room where the baby could sleep when they visited. That never happened. Not one of my children ever spent a single day at Grandma's place.
She had just reached a place where she couldn't do it. She also wouldn't be helped. It wasn't even about accepting the help, it was that she just couldn't part with any of the stuff. It had become a full on mental health issue. As she aged, I know people judged us for not just going in and doing it, but she would cry and yell if you even tried.
When she passed away, I was working full time. It took my six months of going every Saturday at 7am with one of my then adult children and sorting stuff. At lunch time the adult child would leave with a van full of stuff to either donate or take to the dump. Then my husband and another adult child would come and help me for the afternoon, until 7pm.
From one bag, box or cupboard to the next, I never knew what I was going to find. Maybe old clothes, maybe old school work belonging to my own children. She would go through my rubbish bins every week to remove anything she thought I shouldn't throw out. I found two shoe boxes of ping pong balls, why? No idea. I found 100 roll on deodorant containers, all in pieces and completely washed out, just in case we ever wanted to put sun screen in them so the kids could take them to school. Just an astounding array of items.
You can't change your mother, unless she gets some counselling to help her deal with the underlying issues she has. You mum probably isn't the same as mine, but there will be something underlying this issue, even if it's a 'simple' as depression.
Now you have to decide how you will handle it. If I had decided things had to be more even for her to have a relationship with her grandchildren, then there would have been no relationship at all.
It was sad that my kids never got to spend any time at Grandma's house, but in the end, it was mum that missed out, not the kids, because the relationship took place at other venues or at our place. Mum died 6 years ago and they all have very fond memories of her. She and I knew how to press all of each other's buttons, but despite the issues in our relationship, many caused by all of the stuff, she was loved and I'm glad we didn't cut down on visits or her seeing the kids for something that she ultimately really couldn't control.
I have no doubt about how much she adored her grandchildren, despite not having any space at all for them inside her home.