Heat canola oil in a deep fryer or heavy pot over medium heat.
Mix squash, onion, serrano pepper, flour, egg, cumin, chili powder, paprika, salt, black pepper, and cayenne pepper in a bowl.
Drop squash mixture by the spoonful, working in batches, in the hot oil; fry until golden brown, 5 to 7 minutes. Transfer squash fritters to a paper towel-lined plate to drain. Fry the corn tortillas in the hot oil until crisp, about 2 minutes per side.
Layer each tortilla with lettuce, squash fritters, avocado, tomato, and a tablespoon of cotija cheese.
