Little sundresses are colorful, fun, cool, and delightful. But they do not belong at the office.
Dresses should not have an open back or halter back -- even if you wear a jacket or sweater over top of them. Showing your back at work is showing too much skin. Period.
To stay cool at work, wear lighter colored fabrics (they absorb less heat in sunlight) and lighter weight fabrics. Cotton is an excellent choice of fabrics for dresses, tops, and skirts. Suits made of linen and silk are cooler than those made of less breathable materials like wool and mohair.