MOURNER AT THE DOOR By Gordon Lish. 162 pp. New York: Viking. $16.95.

From Huck Finn to Holden Caulfield, ''Call me Ishmael'' to ''So it goes,'' the calling up of home-grown voices, narrators who address us in the unbuttoned vernacular with conversational immediacy, has been one of the impudent glories of American fiction. Backwoodsy or ethnic, laconic or rambling, the most commanding of these straight talkers take a shortcut into the reader's imagination. We seem to be getting the story firsthand, unvarnished and unpackaged, instead of through a literary sieve - especially when the narrator is also at the center of the action.

Gordon Lish's first novel, ''Dear Mr. Capote,'' which was cast in the form of a serial killer's letter to the author of ''In Cold Blood,'' was very much a bravura performance in this informal-monologue tradition. Mr. Lish's nameless New York psychopath unburdened himself in a disarmingly colloquial voice, mixing Runyonesque locutions with garbled cliches (a la Archie Bunker) and casual obscenities. The contrast between the narrator's chatty spiel and his horrific confessions gave rise to mordantly amusing effects. The book's essential flatness could be taken as a skillful reflection of the madman's deformed sensibility. (''Peru,'' a second, less forceful mock-memoir, also tackled the theme of violence alienated from feeling.) Yet, for all the craft and energy on display in this impersonation, one could never forget - not even for one of suspended disbelief's fractional moments - that the voice of ''Dear Mr. Capote'' was being artificially, laboriously produced by a sophisticated ventriloquist.

In Mr. Lish's short stories, unfortunately, not only are you aware of a ventriloquist at work: you keep seeing his lips move. Both ''What I Know So Far'' and this second, even slighter collection offer fragments of nakedly autobiographical free association. Some of the new material - the ambivalent mourning of a father's death, the panicky love for a child growing up too fast -comes with built-in emotional clout. But the first-person voices in these 27 pieces, though fitted out with the soul-baring vocabulary of the analyst's couch, are so mannered, so derivatively styled, as to cancel out all intimacy and empathy