Why have Putin and Kim Jong Un publicly supported Trump? Presumably they want Trump to win, but they must know that they themselves are wildly unpopular in America. The leaders of Russia and North Korea endorsing that candidate is, (I would think to most Americans), a reason to vote against Trump, not for him. Shouldn't Putin and Kim Jong Un rather endorse Hillary Clinton if they want Trump to win?
Obviously I can't speak for Vladimir Putin and Kim Jong-Un but the most likely explanation is that they're not doing this for a US audience. It is much more likely that they are pitching their support for its reaction in their own countries.
For example, President Putin is most likely using this as part of the narrative that the western establishment is trying to restart the Cold War. Something along the lines of
Look, the US people support someone who is friendly to Russia, but the US government isn't, so it doesn't represent the views of the US people.
Additional note to Alex's excellent answer is a secondary consideration: a shrewd leader who is unpopular in USA may very well endorse a candidate they would rather see lose, in hopes that people vote against that candidate just as a rejection of said leader's endorsement (probably, with the spinning/publicity of the event the opposing candidate).
Putin (from experience) perceives Obama and Clinton as weak, considering all the losses they sufferer playing geopolitical chess against Putin for the last 8 years, since that idiotic button Clinton wore.
As such, Trump is at best an unknown quantity, with chances of not being as good for Putin's goals as Obama and Clinton were. After all his stated slogan is "Making America Great Again", which is contrary to both Obama and Clinton's rejection of American Exceptionalism approach; AND contrary to Putin and most Russian's view that the world is a zero sum game and anything making America better automatically is worse for Russia.
And at worst, there's a risk of a second Reagan - a candidate derided by the literati, who will pursue a hardline foreign policy with the same... fervor... that Trump conducted his primary campaign.
But I agree with Alex that the primary consideration, as anything any politician ever does, is likely for domestic consumption. According to polls Trump is more popular than Clinton in Russia by a margin of 20%.
(reasons for the latter vary but I wouldn't be surprised if part of Clinton's unpopularity stems from Russian's hate of Bill Clinton over bombing Serbia. US politicians - or at least Clinton's clique - seem to often forget just what exactly led to Russia's entry into WWI)
To the best of my limited knowledge, Kim Jong-Un has never actually stated he supports Donald Trump. He rarely (perhaps never?) makes these kinds of statements about foreign states' internal affairs. What did happen AFAIK is:
[The North Korean] State outlet DPRK Today published an editorial Tuesday that called the business mogul a “wise politician” and said he could be good for North Korea. “There are many positive aspects to Trump’s ‘inflammatory policies,’” the author of the article wrote, according to a translation from NK News. “Trump said he will not get involved in the war between the South and the North, isn’t this fortunate from North Korea’s perspective?”
So it was a hint through an editorial that Trump might not be combative against North Korea, like the US has overall been for many decades. I'm not even sure that counts as "support for Trump" at all.
PS - Note that the source of this story, the Washington Post, could be seen as somewhat problematic (owned by Amazon's Bezos, CIA links, etc.); and the accusation of "Evil dictators support Trump", to put it bluntly, sounds like part of Hillary Clinton's election campaign strategy.
Both existing answers are great as they cover directly visible aspects of Trump's actions and statements, and how they resonate with the policies of radical authoritarian regimes like Russia, "North" Korea, "Islamic State", Venezuela, and the likes: in economy, foreign policy, religion, and so on.
However, there exist more fundamental reasons of why Trump is seen as a much better candidate for the US Presidency, from the point of the above regimes. These fundamental reasons go even before the economy, military activities, etc. These reasons are about the common worldview.
First of all, let me notice that "unpopular figures" in the question are all radical, aggressive, authoritarians. They build their regimes according to their individual worldviews, unlike building state policies based on consensus of elites, which is much more common in today's world.
George Lakoff's article, Understanding Trump summarizes it very well.
Check each point and see how Trump's worldview is similar to the worldview of radical authoritarians like Putin, Al-Baghdadi, or Kim Jong-un.
The Moral Hierarchy.
The strict father logic extends further. The hierarchy is: God above Man, Man above Nature, The Disciplined (Strong) above the Undisciplined (Weak), The Rich above the Poor, Employers above Employees, Adults above Children, Western culture above other cultures, America above other countries. The hierarchy extends to: Men above women, Whites above Nonwhites, Christians above nonChristians, Straights above Gays.
Religious Obscurantism. For the above reason, both Trump, and leaders of DAESH and Russia show excessive religious "involvement". Simply because God is the Ultimate Strict Father, and the very next level of hierarchy is occupied by the country's leader.
You follow His commandments and you go to heaven; you defy His commandments and you burn in hell for all eternity.
(Obviously, the "North" Korea uses Communism and Juche as a replacement for religion.)
Winning and Insulting.
Consider Trump’s statement that John McCain is not a war hero. The reasoning: McCain got shot down. Heroes are winners. They defeat big bad guys. They don’t get shot down. People who get shot down, beaten up, and stuck in a cage are losers, not winners.
Here, one can recall numerous similar incidents, including FM Lavrov directly insulted a colleague from Saudi Arabia (YouTube) or Putin telling sex-related jokes to female politicians (YouTube, turn subtitles ON).
Direct vs. Systemic Causation.
Direct causation is dealing with a problem via direct action. Systemic causation recognizes that many problems arise from the system they are in and must be dealt with via systemic causation. […] Many of Trump’s policy proposals are framed in terms of direct causation.
And the same thing can be said about the government policies of the above regimes.
Using Propaganda to Use Your Brain to His Advantage.
Unconscious thought works by certain basic mechanisms. Trump uses them instinctively to turn people’s brains toward what he wants: Absolute authority, money, power, celebrity.
Trump shares many aspects of worldview with Putin, Kim Jong-un, and the likes.
So, in Putin's view, it is arguably easier to negotiate with Trump than with someone else.
The US naturally has opposite strategic interests (than DAESH or Russia). Trump, being elected, will certainly follow the main strategic pattern of US interest. However, comparing to other Presidential candidates, we can safely assume that Trump will rely on his own personal worldview more than the others.