The American people dislike him because he lies a lot, advocates for silly and outdated economic and trade policies, makes baseless claims and accusations, picks petty fights with foes and friends alike, makes promises that are impossible to fulfill, is frequently incoherent, threatens America's international, has demonstrated zero leadership skills, lacks understanding of public policies and is only able to act like a showman, and generally lacks the character and termperment to serve as the commander in chief. He has been known as a BS artist -- for good reasons.
Americans who have a brain typically have an unfavorable opinion of him. They can see through the BS and know that this guy is not the answer.