Bring a large pot of water to boil. Fill a large bowl with ice water. Add 2 teaspoon salt to boiling water, then add collard leaves and cook until just wilted, about 30 seconds. Immediately transfer to bowl of ice water. Once cool, transfer to a large kitchen towel and dry. These can be stored in an airtight container between sheets of paper towel for up to 3 days.
In a bowl, combine yogurt, lemon juice, mustard, and ¼ teaspoon each salt and pepper. Fold in celery, scallions, and dill, then add chicken and avocado and gently toss to coat.
Lay collard greens on a work surface, fill each with chicken salad and mango slices, then wrap like a burrito. Serve with tomato-and-cucumber salad.
