@robotrock Let's see: He completely ruined the meaning of facts. He has lied so often and his supporters don't even care. Facts and truths don't matter anymore. He has shown that it is enough to make people feel something in order to win elections. Similar behaviour has led to the Brexit, a catastrophic decision.
He has given up so much in the talks with Kim: Kim went from being a internationally isolated dictator to be seen as Trump's equal partner but Trump has gained nothing in return except for Kim's will to denuclearize North Korea but we don't know what this means in Kim's eyes or when he will start with that. So we have to hope that Kim actually does something.
Trump is sabotaging free trade in the world and has ended trade agreements that were good for the USA (for example NAFTA or TTIP).
He has destroyed the reputation of the USA around the world.
He has shown disgusting behaviour, for example mocking a disabled reporter.
His reaction to Charlottesville have shown how dangerously positive he sees nazis. I could go on longer but I think this should be enough to show why he is a bad president.