Thanks to the enduring power of idiomatic phrases (and Lewis Carroll), many of us still know what "mad as a hatter" means. The Mad Hatter in "Alice in Wonderland" can provide the inspiration for an easy costume to make.
Get the hat. The Hatter's haberdashery is often drawn as being oversized in relation to the diminutive figure of the Hatter; he's almost lost in his hat. A good costume would include a frayed, oversized top-hat.
Add a 3x5 card to the hat band. The Mad Hatter had a paper stuck into his hat with the cryptic slogan "10/6." It's not some secret code: the numbers refer to the cost of a hat in contemporary London.
Get novelty teeth. The Hatter is commonly drawn has a big overbite. You can get this effect with a rubber plate on your upper teeth.
Get loud clothes. The Hatter's clothes are mismatched and brightly colored to reflect his eccentricity. Pants with loud checks and a purple jacket are good mismatches.
Don't forget the tie. The Mad Hatter is often shown wearing a BIG colored bow-tie and a BIG white collar. These accessories will authenticate your costume.
Take a look at the Mad Hatter's famous "Twinkle, Twinkle, Little Bat" monologue in "Alice in Wonderland" to get an idea of how the character speaks. Also take a shot at a funny British accent.