''DEATH OF A PORN QUEEN'' is an account of the short, sad career of an 18-year-old who fled her Minnesota home in 1982, found success in Hollywood's pornography industry, developed a taste for cocaine and killed herself at the age of 20. The question worried on tonight's ''Frontline'' offering, on Channel 13 at 10 o'clock, is how a nice girl like Colleen Applegate could have been transformed into Shauna Grant, star of ''Flesh and Lace,'' and why she should have committed suicide.

Her parents and friends seem perplexed, and the answers ventured by the narrator, Al Austin, who produced the program along with Mike Sullivan and Andy Greenspan, sound as though they were cribbed from teen-age how-not-to books. Mr. Austin is given to observations about Colleen being ''lost,'' ''unprepared'' and so forth.

Much more interesting is the description of the route by which good-looking young women make their way in the porn game. Miss Applegate first answered a newspaper advertisement for ''models.'' We see the frank-talking man who runs the agency explaining to a new applicant that she will be expected to pose in the nude and, if she doesn't object, in ''simulations,'' pretending to be engaged in sexual activities. The pay isn't bad - $100 to $1,500 depending on who's doing the shooting.

The agent was as good as his word, and Miss Applegate made a hit with photographers for girlie magazines. ''I photograph women in an attempt to bring beauty out of their bodies,'' says one of them, who then tells of his concern over the perils facing kids in women's bodies. All the men who worked with Miss Applegate tell how dedicated they were to her best interests.

From magazines, Miss Applegate quickly graduated to hard-core movies. The impresario who named her Shauna Grant promised, ''You have the capability of being an overnight sensation,'' and indeed during her most successful year, she made some 30 X-rated movies with more than 30 partners for the booming video-cassette market, had an abortion, came down with herpes and built up a cocaine habit that exhausted most of the $100,000 she earned and appears to have exhausted her as well.

Her friendly director passed her along to his cocaine supplier, a man twice her age, with whom she moved in. He tells of their happy months together before he was sent to prison for dealing. Referring to her drug habit, he says, ''It's hard for a cocaine dealer to control someone close to you.'' Left on her own and reluctant to go back to making porn flicks, Miss Applegate apparently went to pieces, using up all the cocaine she could find in the process. Her lover, distressed by her disorderly habits, made it clear that she would have to leave his apartment. She shot herself.

The program keeps insisting there's a lesson of some sort here, but it can't quite figure out what it is, other than that 18-year-olds who run away to Hollywood and answer advertisements for models are asking for trouble. The glimpse of the workings of the pornography business, however, is revealing, if not pretty. The producers recommend ''parental discretion.''

Colleen Marie Applegate