It seems to me, in reading the materials of the time, that Joseph was almost obsessed with the idea of the universal sealing of his people into a distinct community and people of God. The dynastic sealings and adoptions are particularly fascinating in this regard. Also, it's hard to imagine anything else that would have gotten the early saints driven into their own Promised Land and allowed them to solidify into their own "ethnic group" for so long.
The following is an over-simplification, but it appears to me that the more heavenly visionary Joseph (the seer) was fixated on the eternal teaching of a universally sealed family of God (the community of Christ), while the more earthly visionary Brigham (the organizer) was fixated on building a new House of Israel (the kingdom of God) through obvious blood connections. Polygamy fits well into both of those visions, and by the end of the 19th Century I think it had solidified both visions in a very real way.
Assuming from the start that polygamy in and of itself is a terrible thing makes it hard to justify in any way; removing that assumption makes it much easier - even if it still is easy to criticize certain aspects or results. I choose not to make that assumption.
So, I simply don't classify it as a mistake. I don't think the implementation was handled very well, and I certainly don't think Joseph understood it perfectly until the last few years of his life (if then), but I don't see it as a mistake. I also realize I am working from the soapbox of hindsight, so I try to keep 1 Corinthians 13 in mind and take as charitable a view as possible.