Place the shrimp in a large pot of boiling, salted water. Remove the pot from the heat, cover and let stand until the shrimp are opaque and cooked through, about 3 minutes. Drain the shrimp, pat dry and refrigerate.
Preheat a grill pan over medium-high heat. Lightly brush the pineapple slices, onion slices and jalapeno with oil on both sides and cook, turning once, until grill marks appear, about 3 minutes on each side. Coarsely chop the onion and pineapple. Seed and chop the jalapeno. In a large bowl, toss the onion, pineapple and jalapeno with the mint and lime juice; season to taste with salt. Add the reserved shrimp and toss.
To serve, spoon the shrimp salad onto the tostada shells.