Yes, the stock market will perform poorly under Trump. Initially it may go up, but over time it will likely become erratic and eventually drop significantly. The stock market does best when investors and the population in general feel confident about the direction we're headed. With Trump, many will lose confidence, and this will negatively affect the markets.
The stock market will likely suffer major losses over the next four years; however, this is not due to the Trump presidency, but instead it is a result of a 8 year bull market. The stock market cannot post record gains year-after-year, without suffering some losses and bear markets. Eventually, the stock market will enter a bear market.
Despite his persona and inexperience, I think Trump is going to ensure that the stock market remains stable. I doubt whether he wants to be known for being the worst President in modern history and one way to prevent that is by keeping Wall Street happy. If share prices fall, that means a company isn't doing well and jobs are at risk. Trump won't like that.
As soon as Trump was elected, stocks started to soar. The stock market is doing better today than it has ever done. People are ending the year with 401Ks higher than ever before. Trump wants to put business back to work and this is a great sign for the economy under his leadership.