Whisk sour cream, dill, scallions, mustard, and lemon juice in a small bowl to blend.
Lightly coat baking sheet with cooking spray.
Place salmon, skin side down, on the prepared baking sheet. Sprinkle with garlic powder and pepper, then spread with sauce.
Bake salmon fillets until each is opaque in the center and flakes easily with a fork in the thickest part, about 20 minutes (to a minimum internal temperature of 145 °F).