Position a rack in the upper third of the oven and preheat to 450 degrees . In a small bowl, combine the mayonnaise, mustard and lemon juice; season with salt and pepper. On a rimmed baking sheet, combine the panko with 1 tablespoon EVOO and 1/4 teaspoon each salt and pepper. Spread evenly and bake, tossing occasionally, until golden, about 5 minutes; transfer to a shallow bowl.
On the rimmed baking sheet, toss the sweet potatoes with the remaining 2 tablespoons EVOO; season with salt and pepper. Arrange in a single layer and bake on the upper rack, turning once, until brown and tender, about 20 minutes.
Meanwhile, in a shallow bowl, combine the flour and 1/4 teaspoon each salt and pepper. Pour the egg into another shallow bowl. Working with 2 pieces at a time, coat the tilapia in the flour mixture, dip in the egg, then coat in the breadcrumbs, pressing to adhere. Place on a baking sheet and bake until cooked through, about 10 minutes. Serve the fish with the dipping sauce and sweet potato fries.