Symbolism, Imagery, Allegory
Remember that 80s song "I Wear My Sunglasses at Night" and how sunglasses became the universal sign for cool back then? Well, Mrs. Das is basically stuck in the 80s. You can't get her to take her sunglasses off.
That's because not only is Mrs. Das too cool for school; the sunglasses keep her aloof and remote from everyone, like she's Coco Chanel or Jackie O. Note, by the way, that her sunglasses aren't just practical; they're stylish: "She was wearing large dark brown sunglasses with a pinkish tint to them" (IM 14). Our guess? They're probably designer shades.
Anyway, so you get the point: the sunglasses are a symbol of her upper class status and her general poser-ish attitude.
That's why it's such a big deal when she takes them off to talk to Mr. Kapasi. He gets to see who she is for the first time: "She lifted her pinkish brown sunglasses and arranged them on top of her head like a tiara. For the first time, her eyes met Mr. Kapasi's in the rearview mirror: pale, a bit small, their gaze fixed but drowsy" (IM 61).
If you're feeling a little underwhelmed about Mrs. Das' big reveal, we don't blame you. Her eyes don't seem like much; they're kind of just pale and beady. Kind of like her personality once she starts talking to Mr. Kapasi and spilling her secrets.
That's the other meaning of the sunglasses. Sunglasses on=distance and detachment. Sunglasses off=self-disclosure, closeness. (Worst-case scenario in this metaphor: mirrored sunglasses.)
Knowing what Mrs. Das is really like, it might actually be preferable for Mrs. Das to keep those sunglasses of on, for her own sake and for Mr. Kapasi's. This is a case where maybe we all need the romantic fantasy much more than the crummy truth.