The country that gave 51% of the vote to Barack Obama in 2012 and is currently giving him a 50% approval rating is not going to suddenly turn around and elect his antithesis, Donald Trump, because he (Trump) "speaks his mind".
Obama's popularity doesn't indicate that Hillary will win. Bill Clinton was pushing 60% approval rating in 2000, yet his boy Al Gore still lost the election.
Gore sidelined Bill Clinton in 2000 because of the Monica Lewinsky scandal and did not embrace his legacy. Bill's ability to campaign in that race was limited. Hillary did not make that mistake with Obama. She touted his record frequently during the primary and he will play a large role on the campaign trail during the general election. And the President is raring to go. He has a legacy to protect and is as offended by Trump as the rest of us.