Do muscle relaxers help with a injured sternocleidomastoid?

Maybe a little. Muscle injuries generally respond well to ice, rest, stretching, and time. Medications like nsaids (ibuprofen, naproxen, etc...) can help decrease inflammation in and around the muscle. Muscle relaxers can sometimes be helpful, but have the undesirable side effects of drowsiness and sometimes disorientation. Consult with your physician to decide what combination of treatments is best for you.