OP - I have not read through all of the replies that you've gotten so far, but I wanted to let you know that I totally get where you're coming from. My parents were the same way, and were buying (what they deemed) "appropriate" clothing. A few years ago, we sent DD to stay with my parents for a few days. She picked the clothing to pack and went. When we talked on the phone the first day, she said my mom had taken her out and bought her a bunch of new clothes. For the rest of the trip, she did not even take them out of the bag. When I picked her up, my mom made a comment about it and I told her she picks what she wants to wear because it's what makes HER comfortable - comfort of the clothing and of the style she puts out there for herself. I told her we give DD the choice to choose her clothes so she can express her individuality and so she can be comfortable - no tags scratching, uncomfortable seams, etc. DD doesn't care where the clothes come from, and has clothes from Target, Meijer,
WalMart, Hanna Andersson, Juicy Couture, DKNY, Kmart, Christian Dior... it's all the same to her - just clothing! She loves the fabrics from Hanna Andersson and American Girl, and the fit of both of those companies too - It's like they were made just for her and her body type.
My parents now GET it. They still buy her clothes on occasion, but have learned to be understanding if DD just doesn't use them (often or at all). This past winter, though, my mom bought her a couple of outfits that DD LOVED, and wears all the time. My mom was thrilled. She thought the clothes were outlandish (by her standards), but knew DD's taste enough to know she would love them, the look and the feel.
I would just try talking to your mom and explain that your kids clothes are a way that they express themselves. Best of luck to you.