Barbara and Stephanie's costumes make no sense as disguises. You can see basically their entire face - nose, eyes, mouth, and chin - AND most of their hair. Their secret identities should hardly be secret!
Well, firstly, how many people in Gotham know either of these two personally, AND know their Batgirl identities well enough to recognize these features? While Barbara interacts with her dad regularly in both identities, its often implied in various versions he knows her identity and just pretends to not know, while Steph only interacted with two people in both identities (Detective Gage and Jordana), both of whom were distracted at the time (Gage by various issues in his life, Jordana by the recent fake kidnapping of her best fiend, and at the time had barely interacted with Steph so she didn't really have reason to recognize her). Secondly, their masks obscure their hairstyles, which is a big part in a person's look and so by changing it, it greatly changes their faces and the chances of being recognized; after all, how many blondes and red-heads are their in Gotham, anyway? It'd be easy to not identify who they are. And, lastly, compared to the Robins and many others who only use domino masks, or nothing at all, their masks do a pretty good job. Clark Kenting plays a pretty big part in this kind of disguise.