3 toasted nori sheets crumbled (about 1/4 cup), divided
Place shallot, shrimp, and 1 teaspoon salt in a medium saucepan; add cold water to cover. Bring to a boil, reduce heat, and simmer gently until shrimp are pink, about 5 minutes. Drain; discard shallot. Transfer shrimp to a plate; let cool. Peel and devein shrimp.
Meanwhile, cook noodles in a large pot of boiling salted water until al dente; drain. Rinse under cold water; drain well. Whisk mirin, soy sauce, vinegar, and ginger in a large bowl. Add shrimp, noodles, shiso, and half of nori and toss to combine. Top with remaining nori.