How serious or harmful is scoliosis?

Depends. This depends on cause of the scoliosis. Sometimes it may be related to an underlying tumor or neuromuscular disorder that may be potentially serious. For typical or idiopathic scoliosis, it is usually a cosmetic issue & occasionally may cause some muscular pain if of a larger size or involving the lower back.. Curves of a large size can affect lung and possibly heart function but these are rare.
Most cases, not. Idiopathic adolescent scoliosis is a the most common type of scoliosis diagnosed in pre-teens and teenagers. It is a curvature of the spine that can range is magnitude. The harm in scoliosis occurs with very large curves (>90 degrees) that can cause respiratory problems. Otherwise lesser curves can be treated with exercise, bracing, or surgery to prevent progression later in life.