Whisk together mayonnaise, Thai sweet chili sauce, and optional hot sauce. Cover and refrigerate for at least 1 hour.
Peel, devein, and remove tails from shrimp if needed. Pat shrimp lightly so they are slightly damp but not wet.
Add cornstarch to a shallow bowl or zip-top bag. Toss shrimp in batches until lightly coated, shaking off excess.
Heat 2 inches of oil in a heavy pan to 350°F to 375°F.
Fry shrimp in batches for 2 to 3 minutes, turning once, until crisp and lightly golden.
Transfer shrimp to a paper-towel-lined plate and let drain briefly.
Toss fried shrimp with enough sauce to coat until glossy, not pooled.
Serve immediately with sliced green onions if desired.
Notes
Use cornstarch instead of flour for the lightest crisp coating. Let the sauce rest in the refrigerator for at least 1 hour so the flavors meld. Do not overcrowd the oil or the shrimp will steam instead of fry. Toss with sauce right before serving so the coating stays crisp.