Chatter about Trump's mental fitness for office has intensified in recent months on cable news shows and among Democrats in Congress.
She also added that Trump would be visiting Britain, as planned, without giving any new details of his trip.
White House officials and Trump's high-profile supporters have launched an effort to raise doubts about Wolff's credibility.
The book claims that for "Steve Mnuchin and Reince Priebus, the president was an 'idiot.' For Gary Cohn, he was 'dumb as shit.' For H.R. McMaster, he was a 'dope.' The list went on".
British Prime Minister Theresa May has dismissed concerns about Donald Trump's mental fitness, saying the USA president acts in what he sees as the best interests of his country.
He was criticising Fire And Fury: Inside The Trump White House, which paints him as a leader who does not understand the weight of his office and whose competence is questioned by aides.
Trump said he never granted Wolff an interview for the book and blamed former adviser Steve Bannon, whom he called "Sloppy Steve", for granting Wolff access at the White House. I guess sloppy Steve brought him in the White House quite a bit and it was one of those things.
Michael Wolff said in an interview that his conclusion in "Fire and Fury: Inside the Trump White House" - Trump is not fit to do the job - is becoming a widespread view. 'I think that would qualify as not smart, but genius....and a very stable genius at that!'
Wolff told NBC on Sunday that "I truly do not want to say the president is a liar", but that he had indeed spoken with Trump for about three hours during and since the campaign.
"Suddenly everywhere people are going 'oh my God, it's true, he has no clothes.' That's the background to the perception and the understanding that will finally end...this presidency". I am the only person to date to conduct a global effort to preach the message of Trump and Trumpism; and remain ready to stand in the breech for this president's efforts to make America great again.
"Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book", Trump tweeted late Friday evening local time.
"This is what is called reporting. You ask people, you get as close as you can to the event, you interview the people who were privy to the event, you interview other people who were privy to the event, you come to know the circumstance as well as anybody and then you report it".