I actually think that both views are valid in regard to this poem. If we have a careful look at the poem, we can see that there is definitely a dream of equality and success in the second stanza, when the speaker, having described how he is separated from others in the first stanza, dreams about a "tomorrow" when he will not be excluded and shut away:
I'll be at the table
When company comes.
Say to me,
"Eat in the kitchen,"
This offers a hopeful dream of the equality and success that can be related to the American dream. However, principally, I think this poem relates to American life and the racism that it featured and authors such as Hughes challenged and raged against. Note the way the poem ends with a clear declaration of how the speaker should have the same rights to sit at the same table as whites:
I, too, am America.
He is demanding that his own status as being part of America is recognised and acknowledged, not just conveniently ignored as he has been shut away from the others in the kitchen. Thus the poem sees to say more about American society at the time of writing, although we can clearly link it in to the American Dream.