I doubt not God is good, well-meaning, kind,
And did He stoop to quibble could tell why
The little buried mole continues blind,
Why flesh that mirrors Him must some day die,
Make plain the reason tortured Tantalus
Is baited by the fickle fruit, declare
If merely brute caprice dooms Sisyphus
To struggle up a never-ending stair.
Inscrutable His ways are, and immune
To catechism by a mind too strewn
With petty cares to slightly understand
What awful brain compels His awful hand.
Yet do I marvel at this curious thing:
To make a poet black, and bid him sing!
Awful  here is used in the old-fashioned sense of  “inspiring reverential wonder or fear.”  In other words, it’s the equivalent of “awe-inspiring” or the older meaning of “awesome.” 
 Here as throughout the poem, Cullen adopts the posture of the puny human being who can’t possibly understand the ways and mind of God. However, he is building toward a giant “Yet”–“Yet do I marvel”–an expression of  doubt  that subtly and ironically challenges the wisdom of God’s plan.