I wouldn't nag. Or take the TV away, tv has nothing to do with artwork or messes. I'd find something that is relative to the situation. For example say everything is going to be cleaned up, and you will help her do it, the 1st time. And then get everything clean. Then maybe have a rule that she is allowed to start 2 projects, but if she starts a 3rd, then she must put away project 1 or 2 first. BUT - and it is a big BUT - she must really put away the project 1 or 2. Think of the actual consequences. No nagging, just once asking her to clean up 1 or 2 or you will do it. You may have to help her clean up the first few times, if she is a hoarder or feels overwhelmed by the mess, but have her do more and more each time. But have a very concrete time limit on when it has to be cleaned by. And then if she does not clean up, then one of those items goes in the trash. If it means her good paints or half-finished drawing she spent two days on goes in the trash, too bad. And they don't magically come back later, because then she will just wait it out. If they are gone, they are gone. That is the natural consequence, imo.
I wish my mom had done this for me as a kid, instead of just nagging at me all the time to clean up and then having a huge blow out and it was such a mess I was lost and did not know where or how to even start cleaning. Plus I never finished much, because I would start something else all the time.
Along the way I made this 3 project rule for myself. I am an artist and I have a million things going, but it does clutter. A. And now I finish many more projects, and it makes me look at things honestly if it is not working out and accept it and get rid of it, or finish. But I find having only one thing out is not enough, or fair, because sometimes projects combine, or I literally am painting 3 paintings at once. So the 3 project rule works for me. I hope it may help your DD.