The question Da-Duh repeatedly asks is whether the narrator has anything in Brooklyn that matches what Barbados has to offer.
When the narrator visits with her grandmother, Da-duh, the latter takes her out to explore the grounds adjoining her house. The "grounds" include a sugar-cane field, an orchard, and a...
gully. In the orchard, Da-duh points out mangoes, sugar apples, guavas, breadfruit, limes, and papaws to the narrator. She is utterly convinced that Brooklyn lacks all the bounties that Barbadoes has to offer.
When Da-duh shows the narrator the sugar-cane field, she further expounds upon her hypothesis that Brooklyn doesn't have any cane fields that produce the sugar its inhabitants so readily consume. She also maintains that the narrator probably doesn't even know where sugar really comes from. All the narrator can do is to explain that she has two cavities and therefore is not allowed to consume much sugar in her diet.
When Da-duh takes the narrator down to the gully, she is especially triumphant. She takes to gloating when the narrator admits, without prompting, that New York City has nothing to match the splendor of the Barbadian woods. It appears that Da-duh is intent upon making a point about nature. She is convinced that nothing green can grow and thrive in New York City. Perhaps in her own way, Da-duh is desperately trying to preserve what she believes are the values of an older generation.