Yes. Being underweight is bad. It increases the risk for osteoporosis. It can interfere with fertility and make pregnancy difficult. More importantly, what is the cause of being underweight? If due to severe calorie restriction, are there vitamin deficiencies? Is it due to illness? A normal bmi is 18-25. Mortality increases outside that range.
Extremes not good. Almost all of the medical lititure across a broad range of patients has shown pts with medically defined obesity and those with significantly lower bodymass index ie underweight pts, have worse outcomes.The obese pts have obvious issues. The underweight pt may be more suseptalble to diseases and less reserve when they do get sick.