Airhead simply means some who really likes airbrushing.
The idea is that this person airbrushes so much that the fumes have ate away the grey-matter between their ears leaving only air.
It’s usually a title someone takes upon themselves.
The term is far more common among T-Shirt and Caricature painters than other industries.
For instance I might tell a t-shirt painter at the fair or flea market “It’s good to see an airhead around.” But I wouldn’t call the person giving me an airbrush tan an airhead–especially not if she’s blonde.
Well, actually, I’d probably use airhead as a corny pickup line on makeup artists and spray tanners. But I’m weird that way. And that hand impression on my cheek hasn’t rattled whatever I have left between my ears yet…