Any one struggling now, there are methods like The Organised Mum Method or FlyLady or youtubers like A Slob Comes Clean or Clutterbug that are useful on practical strategy for managing the house.
I grew up with a hoarder who was just about under control in my childhood, but has slipped into dirt and filth over decades of being a widow and issues like arthritis being a hinderence. She had a traumatic childhood, not limited to being bombed out as a child and loosing everything on more than one occasion. Everything was kept "just in case" yet in the case of things like toys it's been to no practical benefit. Broken toys were unplayable. The young family members that the surviving toys were kept for haven't been allowed to play with things.
I generally had what I needed but it was biased towards what she valued with big blind spots. I had pretty party dresses, but never a correct PE kit. She wouldn't replace things she considered still good; didn't understand why Tinkerbell pants were a problem at secondary school (aside from being too small with elastic cutting in). I spent years flatly refusing to wear the coat I was bought in y8 and failed to grow out of. It was "perfectly good" (and humiliatingly young and out of date) There was no accounting for taste changing with age/ fashion. I wasn't permitted to deal with body hair despite the embarrasment it caused because it hadn't been a thing in her youth. I shaved with an old razor I found lying around until I could save up and buy a ladyshave. Had I been a more rebellious/ fashion minded teenager determined to keep up with trendy friends I would have hit more difficulties.
My childhood wasn't neglectful or squalid like so many PPs have described, and I haven't had trauma from it, but there is a practical effect. I was the youngest in the family and had all the hand-me-downs whether I wanted them or not cluttering up my room. I'd be told "tidy your room" but I didn't know how to actually tidy. Tidy= shove everything under the bed/ in the cupboard. It was a big cupboard, built in, but not actually with appropriate shelving. I've spent adult life learning appropriate stategies to tidying. (I do know how to clean, but tidying is a constant battle)
It's possible DM didn't actually know how to tidy herself. Her approach to visitors was to bag stray stuff up and stash in a spare room. Never sorted again. There are still bags of stuff from the 80s of stuff she wanted at the time and rubbish mixed in, and woe betide you bin it because she hasn't read the fashion feature. She probably hadn't learned in her childhood either, but there was less stuff to live with then aside from the trauma of being bombed out.
Today the house has many unusable rooms filled with dangerous and broken furniture as well as decades of stuff. You go through the 50 year old sofa in the lounge. It is a barrier. She's reached the age of not travelling well, but the house is uncomfortable, univiting and reeks of cat piss/ shit that she can't smell after 60+ years of smoking.
I can easily understand how growing up with these conditions is isolating and traumatising even where parents haven't overlaid other abusive behaviours on top.