Basic Trope: A character (usually female) who normally dresses down dresses up for an occasion, surprising and impressing the people around her (including her Love Interest).
Straight: Alice, who normally dresses in jeans and a scruffy jacket and spurns makeup, appears at a function in an elegant evening dress and makeup, amazing Bob with her beautiful appearance.
Alice, who is normally The Pig Pen, appears in a ballgown, literally glowing with beauty. Her very appearance is enough to bring everyone to their knees weeping with joy.
Ninety-nine percent of the time, Alice scampers around nude and gets all sweaty and grubby. The remaining one percent of the time, she is the best-dressed person for miles around and has the demeanour of an aristocratic Proper Lady.
Downplayed: Alice usually wears jeans, a jacket, and a bit of makeup if she has time in the morning. She appears at a function in an evening dress and more makeup than she usually wears, and Bob thinks she's quite attractive.
Alice rarely feels the need to dress up, and it's an aspect of her that people rarely see, thus making it a surprise when she makes an effort.
Alice is a genuinely beautiful person but normally lacks the confidence to present herself to best effect.
Alice, normally known to dress elegantly in every appearance, shows up in relaxed and slightly scruffy attire, surprising Bob.
Gender Inverted: Bob, who normally has unkempt hair, scraggly five o'clock shadow and dresses in a ratty tank-top and jeans, shows up showered, and shaved, in an Armani suit. Alice is in awe when she sees how handsome he is.
Subverted: Alice dresses up elegantly. No one notices.
Doubly Subverted: At least, that's what Alice thinks; Bob in fact did notice.
Alice's idea of what is 'elegant' and 'fashionable' is quite different from everyone else's; people's reactions of surprise and amazement are not for the reasons she thinks.