Trump should not comment on things he doesn’t understand. It was a mistake for him to link his presidential performance to the stock markets to begin with.
Perhaps it is you or we who do not understand Trump. Trump succeeds as president when he is himself, first and foremost. Being himself is what got him elected; it is what has made his first term in office successful for America and himself; and it is what will get him re-elected. Like you and me, Trump is free to exoress himself, unconstrained by previous presidential or political norms.