Place potatoes and 2 teaspoons water in a microwave-safe dish. Microwave at high until tender, about 4 minutes. Place potatoes on a paper towel–lined plate. Let stand 5 minutes.
Heat 1 teaspoon oil in a cast-iron skillet over medium-high. Add sweet potatoes, bell pepper, salt, and cumin; cook until potatoes are crisp, 6 to 8 minutes. Transfer to plate.
Reduce heat to medium. Add remaining 1/2 teaspoon oil to pan. Crack egg into pan; cook until whites are set, 2 to 3 minutes. Place egg on cooked mixture. Top with guacamole and cilantro.