OK, so there's a dual discussion going on here, one about hoarders and another about untidy housekeepers. My ex was (and is) both. There are paths through the house, with furniture (cabinets, entertainments centers, knick-knack tables, computer tables, etc., etc., etc. stacked double- and triple deep along the walls, and every single one with every square inch of surface stuffed full with "things" -- lamps, knick-knacks, figurines, flower pots, bowls, etc. And the house (or rather, the paths through the house) is atrocious: stained carpet, dirty vinyl floors, sinks in the bathrooms filthy and also stacked full of stuff. Now, part of this probably has to do with her bipolar disorder, which she sought for years to treat with meth and/or cocaine (which is why we divorced, not the untidiness and hoarding, which has gotten ten times worse in the 20 years since we split) but I think she probably had the hoarding and poor housekeeping habits before the drugs, too.
She was an attractive young woman, beautiful, in fact. Now she has let her hoarding and housekeeping habits spill over into her appearance, too. She still has size 2 dresses in her closet (per my daughter -- I don't look in her closet, I can barely stand to occupy the space immediately inside her front door) even though she is about 100 pounds overweight now. How does this kind of thing happen? Poor self-esteem, fear of loss, lack of connectedness, mental health issues (such as her bipolar) and personality disorders. People who do that to themselves are much more numerous than people realize. They need help. If you have any in your life, please encourage them to get it. (My kids and I are still working on my ex. I'm getting to the point that I don't much care anymore, but my kids do. And I have a new relationship with a "normal" woman that I'm not going to put at risk trying to help the former love of my life.)