An NYPD whistleblower who’s been a key witness in the trial of the department’s stop-and-frisk policy is a lousy cop, his supervisor testified yesterday

Officer Pedro Serrano’s “activity does not reflect effective policing,” patrol supervisor Sgt. Stephen Monroe wrote in an October 2012 review of Serrano’s job performance.

Serrano wasn’t questioning enough people or conducting enough stop-and- frisks to combat crime in what is “a high crime area” — The Bronx’s 40th Precinct — Monroe testified in Manhattan federal court.

“He’d ask for supervision on simple jobs,” Monroe said. “It wasn’t what you’d expect from an officer with eight years’’ on the job.

Serrano testified earlier in the trial that another supervisor in the precinct demanded more stop-and-frisks.

It was part of the plaintiff’s argument that the city is demanding illegal, racially tinged quotas of stop-and-frisks of minority men.