Is America ready for Rootboy Slim and the Sex Change Band? Warner Records thinks so, and tomorrow the Washington group that broadened the meaning of bad taste flies to Miami to cut a record. Several of the group's cabaret hits, "You Broke My Mood Ring," "Christmas at K-Mart" and "The Heartbreak of Psoriasis" will be featured.

The man behind the band is crazy-eyebrowed, beer-bellied Foster McKenzie, a 33-year-old son of a Washington landscape artist and a Yale graduate who describes his life as a long-running psychotic episode. (He told the Secret Service he was looking for "the center of the world" when he was sent to St. Elizabeths in 1969 for trying to jump the White House fence.)

Offstage, Rootboy Slim is a sardonic, reluctant star who worries his mother might come to one of his concerts, hear him sing his Tijuana folk song called "Bride of the Burro," and "drop her teeth." Onstage, he is a lecherous, perspiring hulk ("I'll have a sweating contest with James Brown anytime") who once threw beets at an audience, explaining "They weren't on the beat, 'get on the beat,' I told them."

Despite his rough-edged persona, McKenzie is shrewed about his commercial future. He says being Rootboy Slim in Washington nightspots such as the Childe Harold or Psyche Delly is exhausting, and only the prospect of a record contract kept him going. Now that he has the backing - his producer also does Steely Dan's albums - McKenzie will try to follow in the path of other hometown talent that made it big: Emmylou Harris, Roberta Flack and Nils Lofgren. "But some have said," McKenzie admits, "we might have trouble mass marketing ourselves."