What do you call a duck that is addicted to drugs?
A quack head!
My mom must be a duck then...
Okay, so basically the joke is using "quack" as in the sound a duck makes and putting it together with "head" to make "quackhead," which is slang for a drug addict. The joke teller then implies their mom is a duck and, therefore, a drug addict! You are probably also addicted to something...like eating lead paint!