No. Unflattering is an adjective.
A prefix is a word part. It is added to a word to change or modify its meaning. (E.g. the prefix un- added to flattering forms the new and opposite word unflattering.)
Fluorescent lights!
Scalawag
Scalawag.
Dishonesty
Poo bar
showing or representing unfavorably; "an unflattering portrait"; "an uncomplimentary dress".
Smelliest
Because he is afraid you will tell his current girlfriend some unflattering things about him.
An English cricketer - if you are being polite - or some unflattering name if he has just played badly!
Retouching means removing unwanted/unflattering objects, or enhancing certain parts of an image.
Prefix=IN
Corrective makeup is to accent a person's best features while minimizing a person's unflattering features or flaws