Vikram Seth compares the noisy and bustling streets of Kathmandu to the quills of a porcupine. In his description, the streets are chaotic, filled with constant noise and confusion, much like the sharp, prickly quills of a porcupine. This comparison highlights the idea that the streets are not smooth or calm, but rather intense, crowded, and full of life, much like the threatening, defensive nature of a porcupine's quills.