If You Say Bianca Wasn't The Duff? Then U Didn't Get What "DUFF" Meant..
He even says it on the movie, "DON'T TAKE IT LITERAL", Duff doesn't actually mean fat, ugly, (what it stands for). You all need to calm down on the whole "wtf, she wasn't a duff, she wasn't fat, and she was actually hotter than her friends?" topics (LMAO to the last sentence).
He points out examples of Duffs in the movie at the party, and not one of them was fat or ugly. They were just "approachable" compared to their friends. Blanca's friends were hotter, period. Bianca was cute, she was NOT fat and she was NOT ugly, but she was the APPROACHABLE one.
THEREFOR SHE WAS THE DUFF.
Honestly just re-watch the movie again if you're so confused.