By the time Kerouac and his colleagues came of age in the literary scene, Malcolm Cowley (1898-1989) was an established, major figure among literary critics and chroniclers of modernist literary history in the U.S. and of literary radicalism.
As a kind of social historian, Cowley had chronicled the writers of the "Lost Generation" of the 1920s (in Exile's Return, which charts the roaring twenties modernists' move toward political radicalism in the 1930s) and their successors. As literary editor of The New Republic from 1929 to 1944, with a generally leftist position on cultural questions, he played a significant part in many of the literary and political battles of the Depression years.
Kerouac may be thinking of Cowley as so well established by the late 1950s that he dreams of him as an irony - the chronicler of radicalism and experimentalism that had grown crusty. That he dreams of himself as the Cowley figure adds another layer of irony - and of fear and even self-loathing.