Wall Street is happy because there is no mandate. Money does not like uncertainty. A gridlocked government will not accomplish significant change one way or the other, thus creating less uncertainty.
Was there gridlock on Obama’s Iran deal? No. McConnell greased it because that is what GOP donors wanted. Wall Street probably thinks McConnell won’t cave on corporate taxes but they would be wrong about that, too. The Chamber of commerce has a 4 year long to-do list and they will expect results. Gridlock is the last thing that would be happening in the swamp with a Biden Presidency.