My mother now lives with me but doesn't trust any of my decision making. I am in my 70's. She is 93. She won't get rid of anything. Her bedroom is filled with clothes that don't even fit her anymore. I mean clothes piled. It is so very hard to move around in that room. Moving her into my home meant I moved all of my things in and most of her things in.
We sneak things out! Throw them out the window if need be! And deny any knowledge of the deed. Fire safety and fall prevention are the most important aspects of her life right now, and you are the competent adult whom this falls upon.
Don't let her know you threw anything out, and if she cries, so be it. It's like a little child - you make them get the temporary pain of a shot to prevent the big pain like a disease.
Is her bedroom the only room she hoards in? Is there any way you could improve the sanitary conditions in there without insisting on massive removal? Could you bring in inexpensive clothes racks and hang things? Maybe make a game of it. "You have such lovely clothes! Let's arrange them so you can enjoy looking at them. You have a lot of yellow things -- should we put them together on on rack?"
(I am absolutely not claiming expertise in this area. None! I'm just brainstorming based on dealing with dementia.)
Try to get into her world. Try to show that you value what is important to her. You didn't say there are unwashed dishes among the piles, or dirty plastic bags, or moldy bread. If it truly is "just" her clothes and only a hazard because they are spilling in piles, then MAYBE she would be willing to let you/help you tidy it up, especially if she gets attention and pleasant interactions with you as you do it. "Mom, I didn't realize you had so many scarves! Some of these are awesome! Can we put them in a clear plastic box so we can see them altogether? And could I borrow a few? I'd love to wear something of yours!" ... "Mom, I remember when you wore this to Peggy's wedding. I think you were the prettiest guest there!"
Try to show respect for what she values. I'd say absolutely put you foot down if she starts "collecting" things in other parts of the house, but try to let her enjoy her stuff in her own room, while gently getting it organized better and more manageable.
You don't need this hassle in your 70s! Goodness, when do you get to enjoy retirement and your own golden years? But bless you for doing your best with your mother at this difficult time in her life.
Wouldn't it be nice if you could turn her obsession with clothing into a bond between you instead a constant source of friction?
If she had dementia, you won't be able to reason with her.