If by professional photographer you mean the person that makes living by shooting pictures than the answer is no, without undermining overall usefulness and universality of full frame. There are many professional photographers shooting primarily APS-C and Micro Four Third cameras. There are professional photographers shooting medium format. There are photo bloggers relying on iPhone for travel and food photography. It’s not about the gear that makes the photographer professional, it’s more about the skills and using whatever camera that make the job done in the best possible way.