I think that Cullen's work is extremely profound and layered with many meanings. To attempt to quantify it in one judgment is dangerous. Being a complex man himself, it only makes some level of sense that his work would be intricate. I don't sense an outward display of triumph. I think that Cullen is making a statement about being of color in a time in American History where it was more challenging being a person of color than a Caucasian individual. At the same time, Cullen is making a statement that possessing the eye and mind of a poet is also uniquely challenging. In both narratives, the difficulty arises when reality is perceived much differently than from what others behold. The triumph might not be in anything more than articulating this vision, being able to speak of unspeakable pain and agony. Perhaps, Cullen argues, this is where one finds a level of victory. While freedom cannot guarantee any semblance of totality and happy ending, Cullen might be arguing that regardless of this condition, it might be the only option individuals in his predicament possess. For example, in using the myths of Tantalus and Sisyphus, one understands that there is little escape from one's perpetual condition of pain and doubt. In the final analysis, this becomes where victory, not in result but in action, might be present. There is pain and hurt in such a state, but there is solace in being able to exercise a sense of freedom despite such a condition.
To me, Cullen's poem shows an acceptance of fate, but one tinged with regret. I don't really see anger or celebration but more or resignation.
To understand why, look at all the things he compares to the condition of a black poet. He compares it to be being a blind mole, to being Tantalus or Sisyphus, both of whom are being tortured for eternity. Give this, I don't really see how this can be triumphant.
So, to me, Cullen is saying that he doesn't understand why God would make him this way, but the last quatrain and the couplet imply a bit of bewilderment and acceptance more than anger.