a plain Jane
A female who is not considered physically attractive by societal standards. Betty always felt like she was a plain Jane, so she was very surprised when the most handsome boy in school asked her to be his prom date.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a plain ˈJane(disapproving) a girl or woman who is not very pretty or attractive: She was a shy girl, who always thought of herself as a plain Jane.
Farlex Partner Idioms Dictionary © Farlex 2017