Is it unhealthy to make yourself underweight?

Depends how: "ideal weight" is subscience / entertainment. Many elite athletes, especially runners, have light, amazingly efficient muscle and no body fat. This is great and it's fine. If you simply starve yourself in the hopes of improving your definition, and aren't doing healthy exercise, you'll end up disappointing yourself but it's a risk to health only if you're abusing your body. Talk with your dr.