dpolston is right, for this particular use of DOF there really is no right or wrong, just did it create the desired effect. From here I would say it did.
DOF is more commonly used to separate a subject from the background but when the background is the subject that principal goes out the window.
With this use of DOF the facet that makes or breaks it is the focus point that the DOF originates from and that looks fine to me in this image.