Should I keep doing violin even though I'm majoring in art?

Yes!!!! Do what you love. If you love the violin, do it. As long as you have at least one or two ECs related to art, the violin can't hurt. It's also somewhat related to art since playing an instrument is creative like art. I play the violin myself in addition to the guitar, ukulele, steel drums, and viola. I want to major in computer science or engineering so as you can see playing an instrument is even less relevant in my case. I still continue to play my instruments even though they are not directly related to my major because it's what I love to do. I would not be as happy without it even if it hurts my chances for college. You are young, do what you love, and enjoy high school. Planning for college is a very good thing but don't quit an activity that you enjoy for college. Colleges will see you have a passion and they will not reject you simply because you chose to focus on the violin as one of your ECs.

Colleges want to see what you did as a way to support your declared major, but it is not the only thing they focus on. They look at you as a person in general. They are interesetd in your academics, your extracurriculars, your volunteering, your family responsibilities. Of course you usually have one or two stronger extracurriculars, but they are just a small portion of your application anyway. I think if violin is something you enjoy then you shouldn't stop for the sake of your college applications. It doesn't really matter if you are the best. What matters is that you are passionate about it, that you keep improving, that you keep pursuing it. Hard work beats talent anyway. Colleges want to see people who have many intersets and manage their time in order to pursue them all.

Although I'm not majoring in art, nor playing the violin, I can assure you that you should focus on art, your [passion. And have the violin as a hobby that you have. When you apply for college though, make sure you add violin (I'm assuming you're applying to an art college) as another extracurricular, they don't just want to see "art, art, art" they want to see a blend of other things you've done. Hope I helped!