To put a different perspective ...
First, someone who offers to host Christmas for adult children and GC doesn't sound like someone who's lazy to me. It sounds more like someone who could do with a bit of a hand.
Sorry but you sound quite entitled describing yourself as a capable adult person wanting to go home and "chill". If you really want to go home and do nothing op, I suggest you stay in a hotel!
With that attitude, and if you have siblings who share it,, maybe your mum was busy focusing on raising them and you rather than decluttering?
Many peoples homes become cluttered after twenty to thirty years of family life. It's surprising how easily it happens. I am going through my home right now as my adult DC have left home, and trust me, a good 65% of the items that I am getting rid of, are not mine which I didn't buy, want, or use! And yet it's my responsibility to clean, sort, and get rid of them all!
And I don't think it's fair you labelling her "lazy" for not working for forty years because you are judging her by the standards of today and not as they were back then.
It can be harder to maintain high standards at home than it is at work, because it's all a matter of personal willpower, rather than you being alongside people who are working every day. Also, at home, your status as a housewife is pretty low and people don't just obey you like they would a boss! Trying to organise a family is like herding cats sometimes!
I know my home became a lot cleaner and more organised once I starting working three days a week because I responded well to the structure and used my time more effectively. Maybe she had reasons for not working outside of the home that you don't know about?
Also, if your mum is in her sixties and post-menopausal, she may not have had as much energy to host as she did before. Or she may be depressed?
Either way, I think YABVU indeed op!
This answer may be influenced by the fact that some of my adult children pitched up for three festive meals that I laid on over Christmas and not one of them lifted a finger despite my asking everyone to pitch in. Their excuse was that they were tired and stressed by their own jobs or study. None of them are married or have DC! I have put a lot of work in to tidying, cleaning, shopping, laundering, cooking, and clearing up, and now, I'm tired!
Edited to say: I have just read your updates op that weren't included in your opening post. It's apparent that there is a lot more going on than a messy house. Your mum is obviously depressed. Or has an executive function issue. Or feels she has lost autonomy in some way. But my answer is still the same. Why go home to your family home for Christmas and expect not to pitch in, when this situation has obviously been going on for years? If you want a different outcome then why repeat the same pattern? Why not make Christmas happen the way you want it in your own home?