No, direct outdoor storage in sunlight is not recommended. Fire extinguishers are pressurized cylinders. Most cylinders are rated for operating temperatures up to 55°C or 60°C. In hot summer afternoons, direct sun exposure can raise cylinder temperatures and internal pressure close to safety limits.
Always use weatherproof, ventilated protective cabinets with glass viewing doors for outdoor placements. This keeps the unit out of direct sunlight and prevents corrosion from rain.