I can actually view Newgrounds quite well on my iPhone, and it still loads very well. Of course, the only thing you can really do on the site with an iPhone is view and post on the BBS.
Plus, since none of Apple's mobile devices are compatible with flash, I don't think it would be of much benefit. Then again, I think some of the newer Androids can view flash content, so a mobile version may benefit them more.
But still, I don't think there's much need if the website loads sufficiently on mobile devices as it stands. It would be convenient, but not at all necessary.