Plain women and men for that matter are often "introverted" and seek out ways to be in the background rather than stand in the middle of the circle. They want to fly under the radar.
More often than not they are in agreement with those who find them less attractive. A plain girl is not likely to invest a lot of time and money on her physical appearance, wardrobe, or dress in a provocative manner. She doesn't want to compete with other women.
It's not unusual to see a "makeover show" where a "plain woman" is given a new hairdo, contact lens, a form fitting wardrobe and high heels that shocks an audience. A lot of plain women want to go unnoticed. They're uncomfortable with sexual attention.
There are also instances where some women were sexually abused or harassed early on and deliberately downplay their appearance or pack on weight in an attempt to not have men pursue them.
Appearing plain is their version of a possum playing dead.