Whisk together mayonnaise, sour cream and sriracha sauce in a small bowl. Add water, a few drops at a time, to reach your desired consistency. Set aside.
Place a large piece of aluminum foil near cooktop. Heat a dry, heavy skillet (ideally cast iron) 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 wok, giving each shrimp some room to crisp up. Cook, turning often with a spatula until shrimp are pink, 2 to 3 minutes. Add salt, pepper and fresh garlic; toss with spatula just until spices are dispersed and garlic is fragrant, about 1 minute. Remove from heat and transfer shrimp to a plate or platter.
Assemble tacos: 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.