Can Planet Data Optimizer be used on a VMassive Exotic with all three resources at 15xMega Rich, or is this planet at its maximum?

With the exception of the one shot planet shifters and the Kronyn datacube, you cannot improve a resource once it has hit 15x or more.

You can, however, use a PDO to increase Research if the other resources are at 15x or higher as long as the Research is less than 15x.