Place an oven rack in the center of the oven and preheat to 400 degrees F. Spray a heavy baking sheet with vegetable oil cooking spray or line with parchment paper.
For the vegetables: Toss the zucchini, bell pepper, squash, oil, cumin, oregano and salt in a medium bowl. Spread the mixture in a single layer on the prepared baking sheet. Roast until the vegetables are golden and tender, 25 to 30 minutes.
For the tostadas: Brush both sides of the tortillas with the oil using a pastry brush. Heat a 12-inch nonstick skillet over medium heat. Working in batches, cook the tortillas until crispy, 2 to 3 minutes on each side. Drain on paper towels.
For the cream: Mix the pepper, sour cream, lime juice and agave until smooth in a small bowl.
Spoon the vegetables on top of the tostadas and serve with a dollop of the chipotle cream.