Clinton lost last year's presidential election to the insurgent Republican despite having decades of experience in politics.
Tension has been running high for months as the White House's new incumbent and the North's Kim Jong-Un trade threats of war, with Trump dubbing Kim "Rocket Man" and being called a "dotard" in response.
In recent months Pyongyang has carried out multiple launches of missiles potentially capable of reaching the US mainland, and its sixth nuclear test.
"I am worried about some of the recent actions from the new administration that seem to raise tensions. Our allies are now expressing concerns about America's credibility and reliability," she said.
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"Picking up fights with Kim Jong-Un just puts a smile on his face. It's like picking fights with NATO and the EU which puts a smile on Putin's face," she added, referring to the Russian President Vladimir Putin.
Such "cavalierly threats to start a war are dangerous and short-sighted", Clinton said, adding the regime would be "thrilled" to get the "personal attention of the leader of our country".
Without once mentioning Trump by name, she also said she was "very concerned that the new administration is draining the government of the expertise" in diplomacy over the North.
"There are few Asia experts of senior level left at the State Department," she said.
She felt "so responsible" for the defeat, she said, but also blamed multiple factors including sexism, misogyny and Russian intervention she recounted in her recent memoir "What Happened".
Moscow, she said, was "trying to destabilise every democracy".
Trump's presidency has been overshadowed by allegations that his campaign team colluded with Russia during last year's election.
Moscow has faced accusations of state-sponsored cyberattacks on countries including Germany and France to influence election results -- allegations it has steadfastly denied.