Naidu's title "Corn-Grinders" draws upon a metaphor common in the Indian subcontinent, where a grinding mill symbolizes the pitilessness of existence.
As Spagirl writes in her answer, corn here refers to any grain, not just maize. Until fairly recently, grain being ground was a common sight in India. Consumers typically purchased wheat, millet, sorghum, etc. whole and would grind the grains at home using a gristmill or quern. The quern consists of two flat, circular stones. The lower is stationary while the upper has a central hole into which grain is fed. This upper stone is then rotated by means of a vertical handle, grinding the grain as it is crushed between the stones. The flour spills out all around the mill from between the two stones. Bimal Roy's 1963 movie Bandini furnishes a video of the process. The singer is the peerless Asha Bhosle, the composer Sachin Dev Burman; the actress lip-syncing onscreen is Dolly Kapoor.
The ubiquity of such mills has engendered a prevalent metaphor of life itself as a process of being trapped in a gristmill. This metaphor has its Western analogs in the English expression the daily grind and arguably in the ancient phrase about the mills of god, but in its long history on the Indian subcontinent it has a more pessimistic import: anything that exists, exists only to be inexorably crushed. Perhaps the earliest, and certainly the best-known, expression of this sentiment is from the 15th century poet and mystic Kabir (1398?–1518?), who says that the sight of a mill being worked caused him to cry out in anguish.
चलती चाकी देखके दीयो कबीरा रोय
दो पाटन के बीच में बाकी बचा न कोय
chalatii chaakii dekhake diiyo kabiiraa roy
do paaTan ke biich me.n baakii bachaa na koy
Seeing the millstones at work, Kabir wept:
Between the two plates, nothing endured whole.
Here is a 1975 recording of some of Kabir's couplets, including this one at the 1:47 mark. The singer is Lakshmi Shankar, the composer Murli Manohar Swaroop.
Blogger Rajender Krishan points out that the two plates are the earth and the sky. As the sky wheels over the earth in the passage of time, everything between them is ground down to dust.
Naidu's "Corn-Grinders" is part of a grouping called "Folk Songs", and the title alludes to this folk metaphor of the quern. So common is the metaphor that merely invoking it adds layers of meaning. The mouse, deer, and groom are not individual deaths, they are part of the grinding down process of time to which we are all doomed. A similar allusiveness is at work in the video from Bandini: the visual of the prisoner grinding grain into flour tells us that her plight isn't hers alone. We are all prisoners, trapped between the millstones of earth and sky. Shailendra's lyrics do not themselves touch upon the metaphor at all; in the subcontinental context, the visual alone is sufficient to evoke this meaning.
Pioneering Indian poets writing in English, such as Naidu, often sought to bring together the conventions and history of English literature with situations and metaphors that are specifically meaningful to Indian readers. (Aurobindo Ghose does pretty much the same thing in Savitri.) To a reader of English literature, the mouse would evoke Robbie Burns, the deer Thomas Wyatt. To Naidu's Indian audience, the title "Corn-Grinders" would additionally evoke the entire literary, religious, and philosophical symbolism that the gristmill bears in the subcontinent.
Note: It might perhaps be a bit of a stretch to say that Nissim Ezekiel also has this particular metaphor in mind when he mentions the mills of god in "Philosophy", but Ezekiel's intelligence and craftsmanship mean that anything is possible. Ezekiel was not of course concerned with balancing Indian and Western literary traditions the way his compatriots half a century or more earlier were. Writing within an established tradition of Indian poetry in English, Ezekiel could take both traditions as his own without having to negotiate a space for his writing.