"People start thinking behavior that in normal times we would consider completely unacceptable and outrageous becomes normalized," Obama said during a news conference at the tail end of his visit to Laos.
"People start thinking that we should be grading on a curve," he said. "But I can tell you from the interactions I have had over the last eight or nine days with foreign leaders that this is serious business."
Obama has repeatedly warned against the perils of the Trump presidency, suggesting the Republican nominee would be woefully unprepared to take office should he win in November.
When he's abroad, Obama has relayed fears from foreign leaders, who he says question him often about the state of the US presidential contest.
"You actually have to know what you are talking about and you actually have to have done your homework," Obama said Thursday. "When you speak, it should actually reflect thought-out policy you can implement."
Obama: 'Wacky ideas' should be challenged
Obama was speaking just before boarding Air Force One for a long return journey to Washington.
One of his first events when he returns is a solo campaign stop for Hillary Clinton, the second time he's hit the campaign trail for the Democratic candidate.
Obama declared last month that Trump was unqualified to succeed him as commander in chief, and reiterated his view Thursday.
"Every time he speaks that opinion is confirmed," Obama said.
"I think the most important thing for the public and the press is to just listen to what he says and follow-up and ask questions about what appear to be either contradictory or uninformed or outright wacky ideas."
Confident on Gitmo
During his press conference Thursday, Obama insisted his vow to close the naval prison at Guantanamo Bay could be accomplished before he leaves office in January, despite a recalcitrant Congress.
"I expect to work very hard over the next four and a half months," he said. "I continue to believe Guantanamo is a recruitment tool for terrorism."
The White House has proposed transferring detainees both to other countries and to prisons in the United States. The plan has drawn sharp opposition from Republican lawmakers, whose approval would be required in order to move Guantanamo detainees to the US.
"There is no doubt that because of the politics in Congress right now, it is a tough road to hoe," Obama conceded Thursday. "
Obama brushed off angry comments from Philippines President Rodrigo Duterte, saying the bombastic leader has mouthed off before. Earlier this week Duterte's lewd jab at Obama -- calling him a "son of a whore" -- prompted the White House to cancel planned bilateral talks.
"I don't take these comments personally because it seems this is a phrase he's used repeatedly, including directly at the pope and others, so I think it seems to be a habit," Obama said Thursday.
Obama said both of their governments should work together to advance relations between their two countries.
The two leaders interacted briefly on Wednesday night during a dinner for leaders gathered here in the Lao capital for a summit, but exchanged only "pleasantries," according to a White House official.
Correction: An earlier version of this article and the headline incorrectly quoted a remark President Obama made about Donald Trump. Obama called him "wacky," not "petty."