Answer:
The narrator thinks that the white umbrella would represent something unnecessary, superfluous.
Step-by-step explanation:
The narrator knows that her mother does not encourage unnecessary expenses, even if this expense fulfills some of her daughter's wishes and makes her happy. The narrator's mother does not admire American habits in relation to consumerism, the undisciplined taste in money and believes that a white umbrella would be an unnecessary expense, since her daughter already had a raincoat.