I reckon she probably needs support to get it to a level where it's OK for you, and clear boundaries/guidelines of what you're OK with, and it would probably be best if this is not sprung on her, but she has a bit of time to get used to the idea.
So I reckon you could start planting seeds about wanting to help her do a proper sort out in her room and set a date for this, which is far enough away that it won't feel like a terrifying imposition. Then make it clear she will retain creative control, but your hard lines are [food/damage to carpets/avoiding damp/whatever they are]. But that you will help her because you know it's a lot to expect her to manage on her own.
I think it would also help if you have a framework for a plan which is clear in terms of steps, but flexible in terms of what happens within each step, and also let her know this in advance. The steps could be things like 1. Sketch out ideas/troubleshoot problem areas, 2. Take out trash / remove things temporarily, 3. Deep clean floors/walls/repaint. 4. Put furniture back/Ikea trip for anything that could be useful. 5. Put wanted things back in. 6. Deal with whatever is left over.
You might have a sense already about whether it would be best to deliver the whole thing in one go (e.g. printout with the date, the hard lines and offer of support, plus the steps) or whether it would be best to drip drip first the idea of you struggling with the hard line things, or her struggling to manage to sort it out on her own, or just offers of support without time pressure, and then only later present the bits of the plan as "This is something I thought might help us when we go through your room, have a look and let me know what you think later, not now."
Does she like things like Stacey's Sort Your Life Out - could you watch some of that with her and then joke about wanting to do it on her room or how you wish someone would help you do it for your house, and/or explicitly suggest doing a sort of mini version of this on her room, if you have a large space you could lay out her things.
It might also help to look at things like Clutterbug - she is fab for ADHD as she has it so refers to this a lot, but she basically divides people into micro vs macro organisers (ie, do you have the patience to put things away in a detailed way or do you tend to realistically shove things in piles/drawers?) and also visual vs hidden (ie, do you want everything on display because you'll forget it exists or get joy from seeing all of it, or do you need everything hidden for the calm/minimalist look?) then she explains why if you're trying to follow advice meant for the opposite organising type, how it will go wrong and what to do instead.
I also think the Ross Greene approach is fab for figuring out realistic solutions to problems - the combo of where's your (mum) hard line, and what's getting in their (DC) way is great. It can take some practice first of all to not jump in immediately with solutions, also for both of you to trust the process but it can be a useful format.