Cook bacon in a skillet over medium heat until crispy, about 8 minutes. Transfer to paper towels to drain, then crumble.
In a large bowl, whisk together mayonnaise, sour cream, lemon juice, Dijon mustard, garlic powder, salt, and pepper until smooth.
Add the shredded chicken, crumbled bacon, cherry tomatoes, and red onion to the bowl. Toss gently until everything is evenly coated with dressing.
Just before serving, add the chopped romaine lettuce and fold in until combined. Taste and adjust seasoning if needed.
Serve immediately at room temperature or slightly chilled.