No, President Trump's Twitter account has not been hacked — his header image is supposed to feature one of his tweets Photoshopped over a picture of him and various people wearing suits giving thumbs-up signs.
Instead of pinning the tweet ("Director Clapper reiterated what everybody, including the fake media already knows — there is 'no evidence' of collusion w/Russia and Trump"), Trump for some reason slapped it on top of an image that has nothing to do with anything he said. While it's easy to read on mobile, the tweet cuts off after "there is" on desktop browsers. (UPDATE: The image of the tweet was removed from Trump's header image later Monday night.)
Trump was on a tear Monday night, giving his two cents on what former Director of National Intelligence James Clapper and former acting Attorney General Sally Yates had to say during their testimonies before a Senate Judiciary subcommittee. He called the media and news "fake" multiple times, said the "Russia-Trump collusion story is a total hoax" and "taxpayer funded charade," and claimed that what Yates shared with the committee was "nothing but old news." While putting a tweet in his banner is new for Trump, he might consider next time not using one that includes the words "collusion w/Russia and Trump."