Combine the refried beans, frozen corn, poblanos, 1/4 cup cilantro, the cumin and chile powder in a medium bowl. Combine the salsa, tomatoes and the remaining 1/4 cup cilantro in a separate medium bowl. Toss the cheddar and muenster cheese in a third bowl; set aside.
Spread about 1/2 cup of the salsa mixture in a thin layer in a 6-quart slow cooker. Top with about 6 tostada shells, breaking them as needed to cover the bottom. Spread half of the bean mixture over the tostada shells, then sprinkle with 1 1/2 cups of the cheese mixture and 1 cup of the salsa mixture. Repeat the layers (tostada shells, bean mixture, cheese, salsa mixture), then top with the remaining tostadas, cheese and salsa.
Cover and cook on low, 4 hours. Uncover and let rest 15 minutes. Serve with the sliced avocados, sour cream, cilantro and pickled jalapenos.