Bring the pork, beer, garlic, cumin, 1½ teaspoons salt, and ½ teaspoon pepper to a boil in a large pot or Dutch oven over medium-high heat. Reduce heat, cover, and simmer, stirring occasionally, until the pork is easily pierced with a fork, 40 to 50 minutes.
Uncover the pot and continue to cook, stirring occasionally, until the liquid evaporates and the pork begins to brown, 15 to 20 minutes. Continue to cook, breaking up the pork with a spoon, until the pork is browned and crisp, 5 to 10 minutes. Add 2 tablespoons water to the pot and scrape up any browned bits.
Divide the pork among the tortillas and top with the Fresh Tomatillo Salsa, lettuce, cheese, and sour cream. Roll up into burritos.