“It would have been more favourable for his image to shut his mouth, rather than talking nonsense as he had nothing better to say,” she said in a statement released by state news agency KCNA on Friday.
She is the first senior North Korean official to comment directly on what South Korean President Yoon Suk-yeol has called an “audacious” plan.
Mr Yoon, who had first proposed the plan in May, had talked about it again on Wednesday at a news conference to mark his first 100 days in office.
Ms Kim stressed her country had no intention of giving away its nuclear weapons and ballistic missiles programme for economic co-operation. She said: “No one barters its destiny for corn cake.”
South Korea's Unification Minister, who handles relations with the North, called Ms Kim's comments “very disrespectful and indecent”.
Inter-Korean ties have worsened amid a stalemate in larger nuclear negotiations between North Korea and the US that was derailed in 2019 because of disagreements over a relaxation of crippling US-led sanctions on the North in exchange for disarmament steps.
North Korea has been testing missiles at a record pace this year. There have been more than 30 ballistic launches, including the country’s first demonstrations of intercontinental ballistic missiles in nearly five years.
The country test-fired two cruise missiles into the sea on Wednesday, the first such test in two months, Reuters reported.
There are also indications that North Korea is preparing to conduct its first nuclear test since September 2017, when it claimed to have developed a thermonuclear weapon to fit on its intercontinental ballistic missiles.
The US and South Korea kick off their biggest combined training exercise in years next week to counter the North Korean threat.
The North describes such drills as invasion rehearsals and has often responded to them with missile tests or other provocations.