I'm someone who accumulates a lot of clutter & detritus, its the bane of my life. The best I can advise is to strip it all back to absolute basics.
What is actually needed on a daily basis? Then of the stuff which is wanted, what is actually used? Of this, make sure there's no more than 1 of each type of thing. I have to do this on a regular basis.
Also why does she needs big bed? No 12 year old needs a double and if it's simply a large framed single then switch it out for a divan with drawers when you are able to.
I have a no-food-in-the-bedroom rule (exceptions being mothers/fathers day or romantic breakfast in bed between partners or if someone is very unwell) and cups must be brought down morning & evening as part of routine: bring water glass down when coming down for breakfast, collect up days cups & bring down just before dinner so they can be washed up with dinner things).
A place for everything and everything in it's place. No dump-drawers (they're like crack for clutter).
Reduce the amount of things. Lots of parents rotate toys for young children, keeping some in storage and some out. No reason why you couldn't do the same with her beauty stuff.
Skincare should be absolutely basic: cleanser, moisturiser, spf and a face mask for treats. That's it. Dermatologists are finding that pre teens and teens adopting Skincare routines that use more than this damage their skin in the long term, and age their skin much faster. She may enjoy it,but as a parent you get to put your foot down here for what's in her best interests.
You can also implement a one-in-one-out policy for extraneous fun items. Sure, she'll grumble but again, you're the parent here.
Store her art in an art portfolio folder, once a year she reviews what she's done & how she's progressed & keeps the best. Good for her to learn how to assess her abilities & progress if she's artistic.
Ultimately, what you say goes. She might hate it but she's 12 and it's become unmanageable then that's more than enough of a reason to put your foot down and say things have to be slimmed down. After the initial wrench she'll forget 90% of what's chucked out. (I do). She probably can't even tell you half of what she has, just feels she can't get rid when she sees it again (also me).
In my life long battle against clutter, being strict and at times brutal is the only way - it's never as bad as you think its going to be.