Diamond as a company, IMO, is responsible for the uptick in Banks and One Pocket, due to the yearly DCC. As far as overall play improvement, I agree with JV's points on that. If you looked purely at table brands vs player performance, one could actually argue Diamonds made players worse. Diamonds are prevalent in the USA, and almost not-existent anywhere else. Yet, USA players are way, way, way behind the rest of the world. The last time USA players were best in the world (or at least in contention for best in the world) was actually in the 90's when Gold Crowns were still the top tournament and pool room table.
I know this isn't the reason, just like ice cream does not cause more boating accidents

Just pointing out that its impossible Diamond tables did anything to make players better.