In a small bowl, whisk together mayonnaise, sour cream, and sriracha. Add water a few drops at a time until the sauce reaches a drizzling consistency. Chill until ready to use.

Place a large piece of aluminum foil near cooktop. Heat a dry, heavy skillet over medium-high heat until hot. Add one corn tortilla; cook 30 seconds on each side until softened and lightly charred. Transfer to aluminum foil and fold around tortilla to keep warm. Repeat with remaining tortillas.

In a medium bowl, coat peeled and deveined shrimp in cornstarch. Heat oil in a large wok or skillet until nearly smoking. Shake any excess cornstarch from shrimp and carefully add a few at a time to the wok, giving each shrimp space to crisp. Cook, turning often with a spatula, until shrimp are pink, about 2 to 3 minutes. Add salt, pepper, and minced garlic; toss just until garlic is fragrant and the spices are dispersed, about 1 minute. Remove from heat and transfer to a clean plate or platter.

Top one warm corn tortilla with 3 or 4 shrimp and sprinkle with chopped cabbage, radishes, and cilantro if desired. Drizzle with desired amount of shrimp taco sauce. Repeat with remaining tortillas. Serve with lime wedges and sliced jalapeño if desired.