Do you think leadership is more important or less important in today’s flatter management structures?

I do not think that one can make a blanket claim about all types of leadership in all industries or even assume that the management structures in all companies in the world are necessarily flatter than they were at some unspecified period in the past.
A good or bad CEO can either harm or hinder a business. A company founded on innovation or disruption requires leaders with innovative ideas and the willingness and power to pursue unconventional ideas. Amazon, for example, could not have achieved it current position without a leader willing to forego short term profitability to build market share. On the other hand, powerful executives who command huge salaries and sacrifice R&D investment for short term profits to boost their own stock options can harm a company. Both of these are equally true in more and less flat management structures.
On the level of middle management, flatter management structures mean more people reporting to a single manager, and thus the effects of bad and good management practices will impact more people. However, part of the philosophy behind flatter management structures is empowering workers and increasing collaborative rather than top-down management. Thus a good manager in such an environment should be less concerned with "leadership" and more concerned with being a facilitator.