How is the diagonal blurring and smoothing effect in this photograph achieved? I know it was taken on an iPhone (by photographer Karen Axelrad) with a shutter delay app. What post process creates such a look?
I notice 2 things about this image that stand out. It's not clear if they're the same 2 things you're seeing, but what I see is a slight diagonal blurring, and a smoothing effect. The diagonal blurring can be achieved in-camera by moving the camera during a long exposure. (In this case, "long" could still be a fraction of a second, but enough to get just a little motion.) It could also be done in post using a Directional Blur (sometimes called "Motion Blur") filter. It performs a blur along a line.
The smoothing effect looks to me like a median filter. A median filter smooths out similar areas while preserving sharp edges.