There is no first-person narrator in "Goblin Market" like in many other poems. There's no "I." Instead, there's an omniscient third-person narrator like you'd find in most novels or short stories. A third-person narrator usually gives the impression of being more distant from the story than a first-person narrator would because a third-person narrator isn't a character and doesn't participate in the plot. The narrator of "Goblin Market" is no exception. She seems to describe the "Goblin Market" objectively, at least at first. She lists all the goblin fruits for sale and doesn't make any judgments about whether they're good or not. The speaker leaves it to Laura and Lizzie to judge for the reader.
Occasionally, as the poem goes on, the narrator will slip in an adjective that suggests that she's not as objective. For example, she describes Lizzie's advice to Laura as "wise" (142) and Laura's silence as "sullen" (271). And finally, towards the end of the poem, the narrator actually breaks out and addresses Laura directly:
Ah fool, to choose such part
Of soul-consuming care! (lines 511-512)
The narrator calls Laura a "fool" for "choos[ing]" to eat the goblin fruit, even though it meant giving in to "soul-consuming care." The narrator's objectivity seems to go out the window in these lines, which mark the climax of the poem. It's as though the narrator just couldn't keep her mouth shut during the exciting part – she had to throw in her two cents.