But it can help if you are having trouble applying the decal. Not all decal paper is the same, and whilst the thinner the better (as it gets hidden under a clear finish) the really thin stuff can be awkward to apply as it wants to fold over and stick to itself through surface tension as it comes off the backing sheet.

If the decal does fold, then often you'll get a crack in the ink which is visible and you have to remove the decal and start again. If this keeps happening, then applying a couple of layers of clear finish will help stiffen it keep it flat and pristine. But if you manage to apply it without any issues, then there's no need for applying some clear finish first (unless you really want to).