His biggest mistake was in writing polygamy into the Mormon canon. It really was just to justify his own sexual appetites. Smith and later his successor Brigham Young ran their domains as dictators.
Polygamy was/is fundamentally incompatible with western democratic values. It inevitably gives rise to evil behavior and the resemblance of the FLDS communities to Wahabi islamist extremism is no accident.
You are so wrong! Mormon Polygamy was no mistake.
If you have a very small group of people that you wish to make larger, what is the quickest way to do it?
You take a lesson from rabbits and rats. You propagate as many offspring as possible by spreading yourself as far and wide as you can, as quickly as you can.
One mate will not do that, if you wish to reach a viable critical mass! We have proof everywhere that Joseph Smith's plan reached fruition.