What is the mood of the poem "Mother to Son" by Langston Hughes?

The mood of "Mother to Son" is one of optimism in the face of hardship and sadness. 

The poem starts off, it seems, on a note of resignation:

Well, son, I'll tell you:

Life for me ain't been no crystal stair.

A "crystal stair" indicates luxury and delicacy. Hughes contrasts this symbol of privilege with images of roughness:

It's had tacks in it,

And splinters,

And boards torn up,

And places with no carpet on the floor --

"Tacks" and "splinters" indicate pain and hazardous territory. Torn boards symbolize destruction, possibly even the loss of a foundation. Finally, "no carpet on the floor" is a loss of comfort and warmth. The anaphora, or the repetition of "and" at the beginning of several lines, emphasizes the continuity of these deplorable conditions. The lack of change is broken only by a single word, "bare," which creates a stark and lonely image.

The pessimistic tone is discontinued, which is indicated by the use of "but" as a transition:

But all the time

I'se been a-climbin' on,

And reachin' landin's,

And turnin' corners,

And sometimes goin' in the dark

Where there ain't been no light.

Anaphora is used again, but this time to show the narrator's continual motion. Notice, too, that she uses the present tense: "a-climbin'," "reachin'," "turnin'," "goin'." This pattern, too, is broken by a line that evokes an image of a space: "Where there ain't been no light."

Her focus shifts from her narration of experience back to her son:

So boy, don't you turn back.

Don't you set down on the steps

'Cause you finds it's kinder hard.

Don't you fall now --

Anaphora is used once again, though less consistently, with the imperative "don't." Finally, the poem ends with motion:

For I'se still goin', honey,

I'se still climbin',

And life for me ain't been no crystal stair.

"Goin'" and "climbin'" are contrasted with her warnings to her son not to "set down" (a resignation of action) and "[falling]" (a failure to remain steady).