Skinny might imply someone looks unhealthily thin; it's not always a compliment. She was worried he looked too skinny after his illness.
Pro Tip 2/3
Fashionably Slim
'Skinny' can be positive when referring to clothing like jeans, suggesting a trendy fit. He loves wearing his skinny jeans on the weekends.
Pro Tip 3/3
Casual Language
While common, 'skinny' is casual; alternatives may be preferable in formal contexts. In the medical report, they described him as underweight, not skinny.