Not if one's healthy
A doctor can check for digestive, hormonal, metabolic, infectious, or cancerous conditions in a person "looking too thin".
Hopefully, no problems are found. Choosing proper designs of clothing and shoes can help one appear bulkier. To increase muscle mass
, one has to work out with weights. To add a little fat (if one is "skin & bones"), one must eat a larger amount of his healthy, balanced diet