For main character and fading Southern belle Blanche DuBois, appearances are important. They’re also generally fake. Consumed with the need to appear younger and more innocent than she actually is, every personal interaction is a series of machinations and contrivances designed to reveal the truth, regarding both looks and reputation.
Blanche obsesses over her aging appearance because she can’t stand to look at her inner character.
Physical appearance is the most important tool of characterization used in A Streetcar Named Desire.