No drinks or food in bedrooms if not kept clean. If it is clean and drinks/food is allowed they must come down each night before bed time or it will attract mice (my nieces room was so bad they actually had mice in it!!). If they don't come down then they are not allowed again until the rule can be followed.
Make sure she has plenty of easy access open storage, drawers, bin, laundry basket so everything is close to hand to use and there are no excuses.
Tell her calmly her room isn't clean enough for the hamster to live in, it is a health hazard for her and the animal, and move it downstairs. Show concern for both of them and sorry it has to be done for both of them.
Are electrical equipment chargers, tv etc a fire hazard if room too untidy (clothes over chargers etc). If they are then remove.
Don't insist on perfection but don't let it get to the point it is so bad it needs a major clear out. Help her until she gets into the habit of it - for example, say can you put your clothes away in the basket/drawers now and I'll help you by taking this stuff downstairs to be washed/empty your bin for you.
Put everything across calmly as her decision/her choice, ignore tantrums. Any tantrums and it is her decision not to have drinks/hamster/tv in her room because she decided not to keep it clear.
Don't allow friends in (after lockdown) unless room is presentable.
She is 11. It only gets worse so sort it now.