If her crest is heavy - fallen or not - it could be a sign of insulin resistance (IR) which increases the chance of founder. Having a mare with a heavy crest, and having had a horse founder, I would worry far more about the risk of founder than the cosmetics. Our mare did have her glucose and insulin tested so she is IR, on a special low carb diet, eats no grass, and gets a daily dose of Remission, a magnesium and chromium supplement to help prevent founder. She also wears neck sweats nearly 24/7 and is a fantastic all-around show mare. Trimming the mane and swapping it from side to side can also help.