I strongly disliked the ending, but for the opposite reason as you -- I thought it was far too optimistic.
The biggest issue I had was the implication that Coin is the only "bad" person among the rebel leadership, so that killing Coin solves everything. Until Coin's death, the book has been urging the point that the rebels are little better than the Capitol they are fighting: they entomb the workers of District 2, they kill Capitol children, they want to initiate a new Hunger Games. But then Kat kills Coin, and (after a brief interlude) all of that is swept under the rug. Suddenly Kat gets to go home and live out her life peacefully with Peeta. District 12 is being rebuilt. Poverty and starvation are apparently gone.
It feels to me as if the author couldn't decide whether she wanted a powerful ending (Kat escaped being a pawn in the schemes of one empire only to wind up the pawn of another) or a happy one (Kat x Peeta otp!), so she tried to go for both, and the result is a mess.
--
You put your RESOLVE HAT back on, which conveniently is the same hat as your NORMAL HAT.
{Drakeryn}