Eeeew, gross, Hamlet! That’s gross!
But I guess that’s what you’re after…to make “that flattering unction” seem as useless and disgusting as possible. And given that the flattering unction is basically the idea that Hamlet is mad (an idea, by the way, that he came up with) it’s a little funny.
I think what he may be doing here is a little verbal slight of hand – pulling attention away from all that has just recently passed by leaning into the repulsiveness of this image. If he can make her feel disgusting instead of terrified, he’ll have moved her in some way – shaken her out of the just having (not) seen a ghost.