For viewers, I suppose looking at the various sites and how many visitors they attract might give some indication. Sites like DeviantArt and even here may also feature photography and non-3D digital work as well, so it’s not that easy though. I’m not entirely sure how you tease out viewer interest in 3D from artist interest. Are people viewing a 3D comic because it’s 3D or because they like the story, characters etc. – if that makes any sense? How much is a specific preference for 3D and how much is the timeless preference for “good” art? The medium has been around long enough that most of the novelty factor has probably worn off and works have to stand on their own. Mr. Sparky just posted about having one of his works exhibited in a brick and mortar gallery. It would be interesting to know how much acceptance 3D is getting in that type of venue. Not to say that it would be a more valid indicator of popularity by any means.

Probably veering wildly away from the question but what is it about 3D art that would make it interesting to viewers as a distinct medium? It enables a type of ‘realistic’ imagery and the ability to create otherwise impossible or improbable images. Photography can certainly do realism and with digital techniques of photo manipulation, it can create the impossible as well. Airbrush artists were creating uncanny realism decades ago. In terms of what viewers might want to see, I’m not sure what 3D has that couldn’t be achieved in another medium. From the artist’s side OTOH, it may be entirely different. I wouldn’t go so far as to say that creating a top notch pinup with Poser is easier than doing it in airbrush – I’ll leave that judgment to artists. It’s certainly a different process though with its own advantages and drawbacks. That’s getting back to the artists choice of medium and what they’re presenting for viewers to choose from. If they create more good 3D, then the popularity of the medium will increase – not necessarily because it’s 3D. Throw in how the availability of Accessible 3D tools has coincided with the rise of the internet and the ease of online exhibition of those works, yada yada.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken