。
She's wearing a baggy jacket.
Literal
She [topic-は] baggy [genitive-の] jacket [object-を] [is-wearing-着ている].
だぶだぶ is a 擬態語 (state mimetic) for loose, baggy fit — clothing that hangs off the wearer in folds. Belongs to the family of doubled mimetics that describe textures and shapes (ふわふわ for fluffy, つるつる for smooth, ザラザラ for rough). The の after the mimetic turns it into a noun modifier: だぶだぶの上着 ('a baggy jacket'). 着る is the wear-verb specifically for upper-body torso clothing; Japanese uses different verbs depending on the body part — 履く for footwear and lower-body, かぶる for things on the head, する/かける for accessories — and getting them right is a small but instantly noticeable marker of fluency.