Preheat broiler with oven rack 5 inches from heat. Broil poblano pepper on an aluminum foil-lined baking sheet 6 to 8 minutes or until blistered, turning occasionally. Place poblano in a zip-top plastic freezer bag; seal and let stand 10 minutes to loosen skin. Peel poblano; remove and discard stem and seeds. Pulse poblano, onion, garlic, and cilantro in a food processor until finely chopped.
Stir together corn chips and milk in a large bowl; let stand about 5 minutes or until chips soften. Stir in eggs, salt, pepper, and poblano mixture. Fold in beef. Shape into 35 (1 1/2-inch) meatballs (about 2 tablespoonfuls each). Place 1 1/2 inches apart on a lightly greased (with cooking spray) rack in an aluminum foil-lined jelly-roll pan.
Preheat oven to 400°. Bake 10 to 12 minutes or until browned. Transfer meatballs to a large Dutch oven; add enchilada sauce, chicken broth, and 1 Tbsp. sugar. Bring to a boil over high heat. Reduce heat to medium, and simmer 15 to 20 minutes or until meatballs are cooked through and sauce is slightly thickened, turning meatballs halfway through.
Meanwhile, whisk together sour cream, zest, and lime juice in a small bowl. Season with salt, and add desired amount of remaining sugar. Serve meatballs with sour cream mixture, tortillas, and toppings.
MAKE IT AHEAD These meatballs freeze beautifully. Bake an extra batch (or two), freeze, and cook as instructed, adding 10 minutes to cooking time.