The exercise wants the student to transform "Her dress was poor" using a verb form of dress.
Answer: She dressed poorly.
Why is "She dressed poorly" a better answer than "She was dressed poorly" (or poorly dressed)?
First, all of these sentences are a little odd. We don't normally say "Her dress is poor", but we say "She is well/badly/neatly dressed". In "She is well/badly/neatly dressed", dressed is an adjective.
As for the other matter that you pointed out, that "dress seems like a causative verb", it's true that dress usually used in the meaning "to put on clothes" (either on yourself or on someone else). But, the verb dress is also used when we wear clothes in a particular style or for a particular occasion. The definitions 1.1 and 1.3 given by Oxford dictionary should make this point clear:
1.1 [with adverbial] Wear clothes in a particular way or of a particular type:
'she’s nice-looking and dresses well'
1.3 Put on clothes appropriate for a formal occasion:
'we dressed for dinner every night'
So, the best answer is: She dressed poorly.