It is unlikely to change the prints too much, given that the majority of the wobble impacting prints comes from the extruder carriage itself. With that said, the "natural frequency" of the printer frame decreases as you increase the mass. Adding mass at the top (where you did) is an excellent choice, given that it is where the extruder assembly moves. Since the movement of the extruder is more/less inline with the centre of mass (which you just increased), it will likely pass through resonance (its natural frequency) sooner, resulting in a low vibration response. Reducing the effective spring rate of the frame would also reduce the natural frequency, however that would require sacrificing rigidity.
Depending on how you assembled your frame, you may or may not notice a difference. I recommend adding in some crossmember supports at 45 degrees on the sides and back of the printer if you notice the frame deflecting a lot. Otherwise, avoid them, since they increase the natural frequency.
The attached picture shows the vibration response of a system. You'll notice that the response spikes at the natural frequency. Adding in damping (as you mentioned) reduces the effect of the natural frequency, however the response does not decrease to the extent of an undamped system when you go past the natural frequency. Automobiles are designed to pass through resonance almost immediately on startup, at which point the underdamped systems have very low amplitude responses due to low damping.
Lots of words! I hope this makes some sense. Tinker away and you'll find a lot of neat things with mechanical vibrations.