The phrase "mad hatter" refers to someone who is crazy or eccentric, like the character from Alice in Wonderland who wears a big hat and acts in a strange way. Full definition
The enthusiasts, from Sweden, Israel, the United States, Canada and beyond, might not have dressed up as Mad Hatters but they were particularly anxious to crawl round the choir stall seats at Ripon Cathedral. (remonline.com)
Johnny Depp's Mad Hatter in «Alice in Wonderland» is «punked out». (contactmusic.net)