1. Steam fresh corn and zucchini on a perforated rack over boiling water just until zucchini is tender when pierced, about 3 minutes. Let stand until cool.
2. Slit serrano chilies lengthwise into quarters, from tip to stem, leaving chili pieces attached at stem.
3. Pour oil into a 6- to 8-inch frying pan over medium-high heat. Add cumin seed and mustard seed. When seeds pop, in about 30 seconds, add chilies and stir 1 minute. Remove from heat. Cool.
4. In a bowl, mix corn, zucchini, yogurt, and sour cream. Season to taste with salt, then scrape yogurt raita into a salad bowl.
5. Spoon oil with seeds and chilies onto raita. Garnish with bell pepper and cilantro.