If you were to do a search, you would find a lot of nail art on deviantART. It's people who have done the nail art themselves. And who's to say that's not art? A lot of the nail art I've seen is quite creative. It's amazing to see what people can come up with, especially when done freehand.
If they did it themselves I'd say it's considered art. I actually think it's quite impressive if done freehand, I'd never be able to keep my hands that steady. Also, it's pinterest. Not that it's a big deal. Just saying.