Drywall mud is a lot thicker than that.

The best way to smooth a lightly textured wall is with joint
compound and a trowel. Smooth it on and let it dry. Sand to smooth and
prime before painting or plastering. If the texture is heavier then use smooth
patch and apply it with a trowel. Let it
dry, then sand and prime.

For tips, tricks and videos go to http://meodedpaint.com. You can email me with
questions at terry@mededpaint.com or
call the store.

You can get the mud on the wall with a roller -- it will need to be thinned, but you will still need to make it smooth with a trowel for example.