Preheat the oven to 400 degrees F (200 degrees C).
Combine Cajun seasoning, chili powder, paprika, garlic salt, and ground black pepper in a bowl. Sprinkle on each side of tilapia fillets, covering evenly; cover each side with lemon juice. Place fillets on a baking sheet.
Bake in the preheated oven until cooked through, 8 to 10 minutes.
Shred tilapia; place equal portions in each romaine leaf.
Blend avocado, onion, jalapeno pepper, garlic, and water together in a blender to make the avocado salsa verde. Stir in cilantro.
Top prepared tacos with salsa verde and cherry tomatoes.