I never herd the term Duff before and I don't know if it exist in real culture, but it made for a humorous movie.
Bianca Piper is The Duff (Designated Ugly Fat Friend). Though it sounds very cruel when spelled out, abbreviated it just means your not as cool or as hot as the friends you hang around, and if your a teenage girl who was made aware of this by your idiot jock next store neighbor without previous knowledge that you were different from your herd, that could put you on a downward spiral.
This movie is just more proof that teenagers don't change (technology does and social media plays a very large part in this movie), which is why the movie seems formulated. I would not spoil anything if I told you how it ends so long as you have seen a teen comedy before.
I find the topic of the Duff fascinating. The character itself has been seen in every teen movie ever. It's always the main protagonist, the semi-cooler kid that wants the protagonist to party and the third wheel, that now has a name. For the first time in teen comedy history(I think) that third wheel got it's own flick and it turns out the Duff is funnier than the one liners we had it say over the course of time.
Mae Whitman played the character with amusing results.
I totally recommended seeing this film. highly enjoyable