She may not care about the consequence for herself, but there is still a consequence for you. As someone who loves you, she should care about that.
Try focusing on one specific issue, rather than general messiness, and have a formal meeting to solve this problem. Get a clipboard and a nice sharp pencil.

First, you state what the problem is and why it bothers you. Then, you ask her to explain why she does this. Next, both of you brainstorm solutions and write down ALL of them, even the silly ones. Finally, talk about all the possibilities, agree on one, and agree how the new policy will be enforced. Then implement it.
Here's an example from when I was 6 years old: After weeks of "Please put your shoes away.... Your shoes are still in the middle of the floor; please put them away.... Put those shoes away NOW!!!" my mother and I had a meeting. She said that I often took off my shoes and left them in the middle of various rooms when I came home from school, and this was a problem because she sometimes tripped over them, which was scary and painful because she was recovering from back surgery. I had not made the connection between her annoyance over the shoes and her back problems, so I began to take the problem seriously once I heard that. I explained that when I came home from school and found my younger brother in the middle of some enticing play, I was eager to join in and didn't want to take the time to walk allll the way down the hall to put my shoes in my closet, but I did want to take off the shoes because my feet were uncomfortable after a long day at school. Mom hadn't realized that. The solution we eventually chose was to move my shoe storage location from my bedroom closet to underneath the bench by the front door; that way, I could put away my shoes as soon as I came home without going out of my way. This worked out very well, and we BOTH felt that our needs were being respected.

IMO, your 7yo and 5yo both are old enough to pick up after themselves, and your 2yo is old enough to start! The trick is to find ways to make it easy and natural for them, and to motivate them to do it as a way of participating in a respectful and harmonious family

rather than out of dread of disapproval.
Maybe the uniform could be hung on a pegboard or draped over a chair, rather than hung on a hanger which can be difficult for a child? That's my only specific idea for this particular situation without hearing the answer to an important question: WHY does she not hang up her uniform?