Paul Krugman argues that Trump was not Putin's useful idiot. He understood the help that he was getting from the information Russian sources provided to Wikileaks. He also valued it because it helped him the win an election in which he lost the popular vote by 2.8 million. Clinton lost Wisconsin, Michigan and Pennsylvania by less than 1%. Its easy to believe that help from Russia and Wikileaks might have contributed to the 1% loss in the critical rust belt states.
There were useful idiots, however, that contributed to the ascendancy of the The Donald to his throne in the Trump Tower. He got a lot of help from President Obama who chose not to disclose the information that he was provided by the intelligence community. He did not want to use the powers of his office to influence the election. On the other hand, leaders in the Republican Party chose to ignore what they knew about Russia's role in providing information to Wikileaks. They also put pressure on the FBI director to release information weeks before the election that was harmful to Clinton. It turned out that the FBI found nothing new in the emails they were examining but the damage was done. It dominated the new cycle in the weeks before the election.
We can argue that Hillary was a flawed candidate and that she ran a bad a campaign. On the other hand, Donald Trump was the worse candidate for the presidency that ever won the nomination from a major political party or ever won the presidency. He was helped by a lot of useful idiots.