The tissue paper thing is not really diffusing.
What it basically does is scatter the light somewhat which has the effect of lowering the power of light that the flash puts out. This does help at times because automatic settings on flashes/cameras don't always get things perfectly right and a little nudge down can help. But you can get the same effect using manual flash settings or using flash-exposure control and telling the flash to put out a slight lower power level.
What creates softer light is when you enlarge the light source relative to the subject. So a bigger surface area of light makes for a softer nature of light on the subject. A softbox or umbrella; you can even fire the flash at the white wall of a building and have the light reflected off that wall be the light source. All those are examples where the area of the light relative to the subject has been enlarged. For macro you don't need vast softboxes and such because the subject is very small so you can get away with smaller modifications on the light (but of course bigger will still work if that's what you gtT)