Doesn't the fact that He's not a clown make it particularly unnerving that He's in fact the scariest possible clown?

Or, in what sense is God not a clown when He created not only every clown but the entire concept of clowning?

Or, being a little more serious, that constraint isn't compatible with the problem Anselm stated. There's no reason for the scariest possible clown to exist: there's no reason that, of the clowns both real and imaginary, the scariest real one should be scarier than the scariest imaginary one. (I think we could argue quite convincingly that, say, Pennywise has scared more people than any real clown.) So the argument "A clown is scarier if it exists" isn't obviously true.

Or put yet another way, the greatest possible mortal (existent or not) is obviously not immortal, but you could plausibly argue that the greatest possible being is.

If you want to maximize on one axis the most superlative being, though, you can probably get to God. I suspect Anselm's conception of God is the ultimate realization of any axis of comparison we can think of: the supreme delight, the supreme fear, the most resplendent, the most hidden, etc. And in particular I think Anselm's axis of greatness is much more defensible, that is, "A being is greater if it exists" sounds much more like a reasonable assumption (even if I'm not sure I accept it) than "A being is scarier if it exists."