The Securities and Exchange Commission's New York watchdog, under fire for failing to uncover Bernard Madoff's alleged $50 billion Ponzi scheme - despite a dead-on tip by a whistleblower - yesterday tearfully defended herself, arguing that she and the agency did the best job possible. "Why are you taking a mid-level staff person and making me responsible for the failure of the American economy?" an upset Meaghan Cheung, with eyes tearing up, told The Post. "I worked very hard for 10 years to make a career, and a reputation, and that has been destroyed in a month," said Cheung, who...