In a medium skillet over medium heat, melt butter. Add onion;; cook, stirring occasionally, until onion softens, 8 to 10 minutes. Remove from heat. Set aside 1 tablespoon Mexicorn for aioli. Combine remaining Mexicorn, and chiles to combine.
In a large mixing bowl, combine flour, baking powder, salt, and pepper. In a medium bowl, whisk together milk and eggs. Gradually whisk milk mixture into flour mixture just until smooth. Stir in corn mixture and crabmeat. Cover and refrigerate 10 minutes.
Meanwhile, make aioli by combining the mayonnaise, lemon juice, zest, garlic and reserved Mexicorn in a small mixing bowl. Chill until ready to serve.
Remove the corn crab batter from the refrigerator and add enough oil to a large, heavy skillet so it reaches about ¼ inch deep;; heat over medium-high heat. Carefully drop 6 to 8 mounds of batter by tablespoon into hot oil. Cook until golden brown, about 1 minute per side. Transfer fritters to paper towels and repeat with remaining batter. Serve with lemon aioli.