Combine imitation crab meat, tomatoes, carrots, cucumber, serrano chiles, red onion, and cilantro in a large mixing bowl.
Stir in lime juice; season with salt to taste. Cover and refrigerate for at least 2 hours before serving for the flavors to blend.
Mash the avocados with a fork in a medium bowl; season lightly with salt. Spread 2 tablespoons of mashed avocado atop of a tostada shell. Spoon about 3 heaping tablespoons of crab ceviche on top of the mashed avocado. Enjoy!
