The more likely culprit is a terrible financial instrument that was hiding risk. Faced with an environment of small returns and a flat yield curve, some investors got a little too clever and started playing around with some dangerous derivatives related to the "inverse VIX." To make the situation worse, they borrowed money (leveraged investing) to buy these questionable financial products.

• The big picture: https://www.cnbc.com/2018/02/06/why-the-market-sell-off-just-keeps-going.html
• Some detail on the financial instruments in question: https://sixfigureinvesting.com/2014/05/how-does-xiv-work

There are reasons that explain why the stock market flaked out recently, and these reasons have nothing to do with Trump. The stock market drop doesn't make Trump look bad, because he wasn't even a player in this game. He was basically irrelevant.

That said, the reasons for the bull market that we've enjoyed over the last nine years come from events that occurred long before Trump, and it's laughable for him to try taking credit for it.