Mud doesn't seem to be a full block, in terms of the Y coordinate. I noticed while working in my swamp base that if you're standing on a mud block:

• You can't then walk up stairs or slabs next to that block. It's as if the stair or slab is more than a block tall (and thus can't be walked on)
• When you step onto the mud block, your Y coordinate decreases by 1, as though you had fallen a block (I think that's why the slab and stairs don't work). It looks visually like a whole block, same as the dirt and stone blocks next to the mud, but acts like a slab or as though there just wasn't anything there.