Preheat the oven to 450 degrees F. Spread the poblanos on a rimmed baking sheet and broil, turning occasionally, until blistered and blackened, about 10 minutes. Transfer the peppers to a large bowl, cover tightly with plastic wrap and let steam for 15 to 20 minutes. Carefully peel the poblanos and discard the skins. Cut a slit along the top of each pepper and gently scrape out the seeds, being careful not to tear open the poblanos. (It’s okay if some seeds remain, but do not remove the stem.) Wipe out the bowl and lower the oven temperature to 400 degrees F.
In a medium skillet, heat 1 tablespoon olive oil. Cook the onion over moderately-low heat until tender and golden brown, about 10 minutes. Add the garlic and cook until fragrant, about two minutes more.
In the bowl of a food processor, puree the crushed tomato with the chipotle chiles in adobo, onion, and garlic until smooth. Season with salt and pepper.
In the large bowl, mix the chicken with the shredded zucchini, corn, ¾ cup of the Greek yogurt, ⅓ cup of cheese, ¼ cup of the mint leaves, and ½ cup of the tomato-chipotle sauce. Season with salt and pepper.
Spread the remaining tomato sauce in the bottom of a 9-by-13-inch baking dish. Carefully fill the poblanos with the chicken mixture and arrange close together with the cut side facing up in the baking dish. Sprinkle with the remaining ⅓ cup cheese and bake until bubbling and heated through, about 35 minutes.
Meanwhile, in a medium bowl, whisk the remaining ¼ cup of Greek yogurt with 1½ tablespoons of water and season lightly with salt. In a separate medium-size bowl, gently toss the avocado with the pepitas and lime juice. Press a piece of plastic wrap on the surface until ready to serve.
Transfer the chile relleno enchiladas to plates. Drizzle with the Greek yogurt crema and top with the avocado relish. Garnish with the remaining ¼ cup of mint leaves.