Machine Poetry-
"The following may not apply," as one disclaimer has it.

The song "Oh Ladies All" seems to be a legimate minstrel song, perhaps inspired by the published non-dialect song, "Oh Gentle Ladies All!," 1849, words by Fitzgerald and music by James Bellak (sheet music at American Memory), http://memory.loc.gov/music/sm2/sm1849/461310/002.gif

There is a poem, "Machine Poetry," about sitting down, turning the crank and grinding out poems with no meaning. Did Emmett make a remark about this, or did someone say it about his rhymes?
I can't find the phrase with relation to Emmett specifically.

Here are the first two verses of the poem "Machine Poetry," by Ellen P. Allerton (pub. 1896):

MACHINE POETRY

I sit me down to make a batch of rhymes;
I've nothing in particular to say;
'Tis just to turn a crank and count the times-
Such poetry is ground out every day.
2
The papers teem with it, why shouldn't I
Help swell this tone that current poets sing?
'Tis neither soft and sweet, not grand and high,
And has no meaning- just an empty ring.

Machine Poetry

Seems to apply to much of minstrel and tin pan alley rhymes!