Preheat oven to 400° Fahrenheit. Line baking sheet with aluminum foil.
Place salmon fillets on baking sheet. Pat salmon completely dry with paper towels, then liberally season all sides of fillets with salt and pepper and drizzle avocado oil over tops of fillets.
Place baking sheet in preheated oven. Bake salmon 8 to 10 minutes, or until salmon reaches 125° Fahrenheit internally.
While fillets are baking, add mayonnaise and sriracha to small bowl. Whisk vigorously until ingredients are fully combined, then set bowl aside.
Add soy sauce, rice vinegar, grated ginger, garlic, and red pepper flakes to second small bowl. Whisk vigorously until ingredients are fully incorporated. Set aside.
When salmon fillets are cooked through, remove baking sheet from oven.
Fill serving bowls with 1 cup cooked rice per bowl. Top rice with baked salmon fillet, then garnish with cucumber slices, kimchi, and avocado slices. Drizzle sriracha mayo and soy sauce mixture over salmon and rice. Serve rice bowls immediately with nori sheets if desired.