I don't remember ever feeling underdressed, though.
You could wear a feed sack at a formal dinner and not look underdressed.
She skirted the group, feeling underdressed and frumpy in her faded wrap, jersey-knit dress.
She was underdressed and fat.
Seriously cold outside so had to have another stout or two before sprinting underdressed, freezing but merry back to the Randolph.