Umm ... NO. Impedance by itself has nothing to do with whether an external amp is needed or not. Typically, high impedances require an amp because small portables/sound cards simply do not have the voltage swing necessary to power a high-impedance set of headphones. However, low impedance phones that are very inefficient will also require an amp.
It's more accurate to state the impedance defines the type of amplifier required (voltage or current), while efficiency really determines how much amplification is needed.