If one is looking at The Crucible as a tragedy and considering whether or not the genre itself is inherently optimistic, I think the answer is yes it most definitely is.
The Crucible was meant to be an examination of McCarthyism and other examples of the strong preying on the weak and using some form of political or religious prejudice to magnify their power. In this very examination the play is suggesting that the opposite is in fact possible and is also desireable when compared to the ugly tragedy portrayed in the story or the play.
One can also observe the optimism in an examination of Proctor's decisions that lead to his death. He is absolutely unwilling to compromise his principles as he takes great pride and joy in his convictions even if they will lead to his death. This willingness to stand up to the moral authority and to maintain integrity is also something that portrays humans, even tragic heroes headed to their deaths, in a positive and powerful light.
Proctor sees the world in a moral light and is willing to stick to those morals regardless of the consequences. And though it leads to his "tragic" death, the heroic part of his actions are what shine through in the play as is often the case in many tragedies.