You are a girl, very thin and don't exercise. But the arm always has muscles. If it's really natural, there's nothing you can do. Muscles are not very ugly. Having muscles looks healthy. So are girls. Since it is natural, why try to get rid of it? That's a very bad idea. Sometimes we have to let nature take its course. As long as you are healthy. Growing a little muscle is not a problem, nor will it affect your beauty. So you don't have to worry. If you really feel ugly, buy some clothes with slightly longer sleeves when you wear clothes. Buy some clothes with floating sleeves. Beautiful, beautiful. This will cover your arm. I hope my answer can help you.