This is an excellent question! Like many works of the "modernist" period in American literature (which covered roughly the first half of the twentieth century), Cummings' poem satirizes the modern obsession with material "progress." Many writers at the time believed that modern people were so obsessed with making money and accumulating wealth that they had abandoned an interest in more lofty, more idealistic values.

It seems signifcant that the first adjective used in the poem is "busy" (1). This word implies the modern emphasis on work of all kinds, including physical labor in industrial settings but also including the work of appropriately named businessmen.

Therefore, by asserting that "Progress is a comfortable disease," the speaker of Cummings' poem seems to imply that an obsession with material progress is a kind of spiritual or psychological sickness.  It can make people financially "comfortable"; it can seem "comfortable" to those (especially the wealthy) who benefit from it; but ultimately it can also be somewhat soul-destroying.

In "pity this busy monster, manunkind," how does Cummings criticize scientific "progress"?

In the poem, Cummings draws a clear distinction between the natural world and what he sees as the artificial world of science and technology created by mankind. As he writes, "A world of made / is not a world of born." Science and technology are interpreted as the means through which man has perverted nature, creating a world no longer worth inhabiting: "listen: there's a hell / of a good universe next door; let's go." The idea of "progress" then assumes ironic significance. It does not represent the improvement or enhancement of human life through applied science; scientific progress is instead a "disease" that corrupts the body of mankind. It is a "comfortable" disease in that mankind is unaware of its destructive properties and feels blind intellectual pride while busily subverting nature.