Mix together the yogurt, garlic, cilantro, 2 tablespoons olive oil, 1/2 teaspoon salt and a few turns of pepper in a medium bowl and set aside.
Preheat the oven to 425 degrees F.
Toss the sweet potatoes with the cinnamon, 2 tablespoons olive oil and 1 teaspoon salt and spread evenly on 2 half-sheet pans lined with parchment. Bake, tossing occasionally, until golden brown, 25 to 30 minutes
Transfer the sweet potatoes to a serving plate, then dollop with the yogurt (serving any remaining on the side), drizzle with sriracha, and sprinkle on the pine nuts, remaining cilantro, a few turns of pepper and flaky salt and serve.