Is this a serious disease or it isn't? pectus excavatum

Not always: Can be purely cosmetic and be made fun of by peers/opposit sex. Can affect exercise tolerance if severe and compressing the heart between the sternum and the spine.

Pectus Excavatum: Pectus excavatum, also known as sunken or funnel chest, is a congenital chest wall deformity that creates a concave, or caved-in, appearance in the anterior chest wall. It can be mild or severe. Worsening of the chest’s appearance and the onset of symptoms are usually reported during rapid bone growth in the early teenage years. When severe, patients develop cardio-pulmonary symptoms.