I deal with nutrition and not genetics but I do think that hybrid vigor applies to breeding two genetically different individuals, this can apply to two different breeds within the same species.
The problem with designer breeds is that one generation does not constitute a new breed. I'm not sure about dogs but in cattle you have to have at least 5 or 6 generations before the offspring are considered close enough genetically to be a new breed. When people cross two breeds, like a labrador and a poodle, you can never be certain what characteristics you will get. From the first generation, if you put two adult dogs side by side it is unlikely you could recognize them as the same breed. To make long story short a cross from the first generation, at least from the animal science perspective, is not a new breed.
In my opinion it isn't very responsible to breed indiscriminately because something looks cute. Designing a new breed with a specific working purpose and end goal in mind is something I don't have a problem with but I just don't think that most designer dog breeders have the knowledge to do this appropriately.