The poem's rhyme scheme is irregular, to say the least. Cummings was renowned for his experiments in rhyme and so it's not too surprising to see them on display here. Such examples include

We doctors know
a hopeless case if --- listen: there's a hell
of a good universe next door; let's go (emphasis added)

The rhymes are only occasional, and when they appear they provide a counterpoint to the poem's loose-limbed structure—which perfectly conveys Cummings's contempt for the idea of human progress and the arrogance on which it is based.

Had Cummings resorted to a more conventional poetic structure, complete with neat rhymes and rhythms, then he would've undermined his own message. It would've been strange indeed to read a poem damning human beings for acting as if they're god-like and omnipotent that was written in a polished, formal style.

On the contrary, Cummings wants to show "manunkind" at his very worst: in all his arrogance and hubris. And what better way to do that than to use a zig-zagging structure with only the occasional rhyme peeping through the chaos?