Preheat the oven to 375 degrees F (190 degrees C).

Place chicken into a frying pan over medium heat. Sprinkle with taco seasoning and cook until no longer pink in the center and juices run clear, 5 to 7 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).

Spread a thin layer of refried beans onto each tostada shell; sprinkle each with Mexican cheese. Divide cooked chicken on top of cheese layer. Sprinkle tomato, red onion, and bell pepper evenly on top of each tostada. Place tostadas on a baking sheet.

Bake in the preheated oven until crispy, 10 to 15 minutes.

Remove tostadas from the oven. Sprinkle with cilantro and spread salsa and sour cream on top.