Cook the sushi rice according to package instructions. If desired, gently stir in rice vinegar, sugar, and salt while the rice is still warm. Let the rice cool slightly.
In a small bowl, whisk together the mayonnaise and sriracha until smooth. Adjust the amount of sriracha to taste.
Divide the cooked rice evenly among serving bowls, spreading it into an even layer.
Top each bowl with the chopped imitation crab, diced avocado, cucumber, and sliced scallions.
Drizzle the spicy mayo over the bowls and sprinkle with sesame seeds.
Serve immediately, or refrigerate for later if preparing ahead.