The Polaroid camera could be significant for a number of reasons. First, the Polaroid was a relatively new invention in the context of the story. Immediately it denotes technological progression and therefore a separation of on generation to the next (here, mother and daughter). Along the same lines of separation, one major theme throughout the story is the clear disjointedness of the mother-daughter relationship, as evidenced through Dee and her mother, and through the motif of eyes and eye contact used throughout. As Dee takes pictures of her mother there is a symbolic element of her "seeing" everything all together. She takes "picture after picture" and "she never takes a shot without mak' ing sure the house is included." Ironically, though she takes several pictures of her mother and what once was her home, Walker makes a very distinct point that Dee doesn't really see her mother nor her home for what it is. The Polaroid, therefore, an an aid which helps mask this view.
Finally, the Polaroid could be further scrutinized symbolically when you consider that pictures are developed immediately. Rather than a reflection in a mirror, everyone who looks at a the picture a Polaroid takes has the opportunity to see the same thing at the same time, and relatively soon after the picture is taken. This could speak to the stark irony in the fact that these two women, who should very well be looking at the "same picture" of things, are not seeing anything eye-to-eye.