Bob's right; it's not easy to do 3D smoothing, but just in case you really want to do something, there are a couple ways to get something smoother on the screen.

If your dataset is to be displayed as an overlay dataset, you can change the warp-on-demand interpolation to be cubic instead of nearest neighbor (the default). The overlay and threshold interpolation to the underlay grid is controlled in the "Define Datamode" menu.

The slab projection methods in the Display menu of the image window allow mean and median over multiple slices, effectively smoothing the data. You can also use the "Tran 2D" options to choose a median filter within the 2D slice displayed in the viewer. You can even combine the slab projection and the Tran 2D to get a 3D smoothing of sorts. The sharpen option is the only filter that works directly in the image viewer at multiple levels, but it sharpens and doesn't blur. Each time the 's' key is pressed, the sharpening cycles among the 9 levels of sharpening.