There's no way of knowing who "invented" or "started" the idea. How would you define "art"? It could be anything. I could scratch my hearing aid and claim that as my art. Another person could put a couple nail polish drops on it and claim that as art too.
I see nothing wrong with putting stickers on your hearing aid.
There are a few things you should consider though.
1.) Make sure it's not the kind of sticker that will stain anywhere. For instance, if the sticker was in contact with your skin and it stains, it could leave a mark on your skin. (I'm sure it can be washed off in the shower or bath.) It could leave a mark on your hearing aid. It depends on the sticker.
2.) Make sure it doesn't cover any important parts of your hearing aid. If you ever needed to have your hearing aid looked at and it covered something they needed to access, it can complicate things for them.
3.) Is the timing right? Are they hearing aids for work? Wouldn't look good having weird stickers on your hearing aids if you were working in a professional environment.