(as|) mad as a (hatter|March hare)

used to refer to someone who is in a very chaotic state of mind and is behaving very abnormally

The idiom "mad as a hatter" dates back to the 19th century and is associated with the hat-making industry. This phrase is used to describe someone who is acting extremely eccentric, irrational, or mentally unstable.

Ever since he lost his job, he's been acting mad as a hatter, talking to imaginary friends and making nonsensical decisions.
If she continues exploring her creative ideas, her art might become as unique and brilliant as that of someone mad as a March hare.
In the past, people sometimes attributed eccentric behavior to being as mad as a hatter, unaware of the toxic effects of mercury exposure.
The artist's latest work is as bizarre as if he were mad as a hatter; it's hard to understand the meaning behind it.