I think there absolutely should be a certification process for Christmas Decorators. instead of interior decorator, perhaps an "exterior decorator" . There should be liability and workmans comp insurance, specific for electric and roof REQUIRED. You should have tiers, such as apprentice, journeyman, master, etc, based on years of experience that is checked by references. If your state requires mechanical contractor liscense for jobs over a certain size, then you must hold that BEFORE you can get your certification.
This certification should mean something, not just for folks who want to pay 35 bucks to sign up.