Is there an English idiom/metaphor for "mind boggling", only more so?

Solution 1:

There doesn't seem to be an existing English idiom that does exactly what you'd like it to. However, based on Hank D's mention of "the mind reels," and JPmiaou's excellent suggestion, I'd offer one of these two:

The mind reels ... and loses the fish.

The mind reels ... and falls flat on its [butt].

Solution 2:

In British English, a crazy (or extremely foolish) person can be called bonkers, but if that person is particularly "loony" and should be locked up (in a figurative sense) then they are stark raving bonkers.

• Any parent that calls their baby Lucifer is not just bonkers, they're stark raving bonkers.

bonkers
[PREDICATIVE] informal, chiefly British
Mad; crazy:
You’re stark raving bonkers!

raving
(usually ravings)
Irrational or incoherent talk:
the ravings of a madwoman
_{Oxford Dictionaries}