Preheat oven to 200°. In two batches, cook bacon in a large cast-iron skillet over medium heat until crisp and brown, turning often, about 7 minutes. Transfer bacon to paper towels to drain.
Season trout with 3/4 tsp. each salt and pepper. Heat bacon drippings in skillet over medium-high heat. Lay 1 or 2 trout in skillet and cook, turning once (use two spatulas), until golden and cooked through, about 4 minutes total. Transfer fish to a wire rack set over a rimmed baking pan and put in oven to keep warm. Repeat with remaining trout.
Increase heat to high. Add tomatoes to skillet and spread in an even layer. Cook, without stirring, until they just begin to char, about 1 minute. Stir in garlic and cook until fragrant, about 30 seconds more. Remove skillet from heat.
Divide trout among four plates and top with tomato mixture. Lay 2 pieces bacon on each trout. Toss arugula with oil and remaining 1/4 tsp. each salt and pepper, then heap on top of bacon.