Cook the rotini pasta according to package instructions until al dente. Drain and rinse under cold water to cool.
In a large bowl, combine the cooled pasta, diced cucumber, cherry tomatoes, and red onion.
In a separate bowl, whisk together the mayonnaise, sour cream, lemon juice, garlic powder, dried dill, salt, and pepper until smooth.
Pour the dressing over the pasta and vegetables, and toss gently to combine.
Cover the salad and refrigerate for at least 30 minutes to allow the flavors to meld.
Before serving, give the salad a gentle stir and garnish with fresh dill if desired.