I'd have to say that if the nickname results in anything at all, it's Janie's search for her own self. She earns this nickname growing up amongst the white children of the family for whom her grandmother was a live-in servant. One day, she sees a picture of the children, & doesn't recognize herself:
So Ah ast, "Where is me? Ah don't see me" (9), "Dat's you, Alphabet, don't you know yo' own self? Dey all uster call me Alphabet 'cause so many people had done named me different names.
The question "don't you know your own self?" is key to Janie's search for her identity throughout the novel. This becomes the basis for her life, moving from man to man, trying to find out who she truly is. The last part of this quote is important too. She notes that people had given her so many different names, & that is what she remains to each person she encounters. She is something different to each person important in her life: to her grandmother, she is something to be possessed and sold off for protection. To Logan, she is another hand to help with chores. To Joe, something to be owned and shown off, like a piece of art. Only to Tea Cake is she a partner and close to an equal, and she loses this when Tea Cake dies. But that allows her to come into her own, and claim the true Janie.