To make the filling: Heat the oil in a large skillet over medium heat. Add the onion and cook, stirring occasionally, until softened, translucent, and just beginning to brown, about 6 minutes. Add the poblano rajas and allspice; stir, cook for about 2 minutes, until the mixture is fragrant and the rajas have darkened a bit. Add the corn and cook, stirring, for 1 to 2 minutes. Stir in the squash, salt, and the pepper and cook until the squash is barely crisp-tender, about 3 minutes. Turn off the heat.
To make the sauce: Place the tomatillos and garlic in a medium saucepan, cover with water, and bring to a boil over medium-high heat. Reduce the heat to medium and simmer until the tomatillos are thoroughly cooked and their color has gone from light green to olive, about 10 minutes.
Drain the tomatillos and garlic and transfer to a blender. Add the salt, avocado, and chopped mint and puree until completely smooth.
Place a crepe on a plate and spoon ¼ cup of the filling across the middle. Roll up like a chubby enchilada and place seam side down on a serving platter. Continue with the rest of the crepes and filling.
Pour the sauce over the crepes, garnish with mint, if desired, and serve.