In network TV, producers never try to improve a hit. If the public buys it, that's it. The only reason "Mork and Mindy" has evolved into such a genuinely and hearteningly entertaining show is that the producers hitched their wagon to a comet -- the gifted Robin Williams, who has been as busy behind the camera as in front of it and has turned a mere hit into a high-comic work of honor.

The producers, meanwhile, are doing what successful producers always do -- trying to develop additional hits, not improve existing ones. And so the Miller-Milkis gang at Paramount presses its buttons, the conveyor belt lurches forward, and out pops "Angie," yet another working-class sitcom "created" by Garry "Happy Days" Marshall and premiering on ABC (Channel 7) tonight at 8:30.

The program is a fitfully engaging mediocrity blessed with a young actress named Donna Pescow in the title role; she's a spunky little turnip, a Karen Valentine with moxie. She plays a self-deprecating waitress who meets the pediatrician of her dreams, falls in love and then learns to her horror that he's positively greasy with money.

Funny Italian mama (Doris Roberts, who never fails) learns to her horror that her future son-in-law isn't Italian. And the not-very-funny snob sister of the rich doctor learns to her horror that her brother is marrying down. "Oh drears," she says, to the laugh track's rollicking delight.

The basic cast is augmented by Debralee Scott (of "Mary Hartman, Mary Hartman") as Angie's sister, and Diane Robin, who, in the role of waitress DiDi, out-Shirleys Laverne and out-Lavernes Shirley. This is the kind of sidekick you really would like to kick in the side.

Writers Alan Eisenstock and Larry Mintz and director Howard Storm have got situation comedy down to a humorless science. Their script is three parts joke to one part sentimental sigh, like this: joke-joke-joke, sigh, joke-joke-joke, sigh. The rhythm gets to be so lulling you could cook eggs by it. But Pescow as Angie is attractive and vaguely endearing, so people will like her, and they will watch the show, and Miller-Milkis productions will have another hit without having exerted one more snyapse than necessary in their shrewd corporate brain.

"Angie" is supposed to be taking place in Philadelphia -- the way "Laverne and Shirley" is supposed to be taking place in Milwaukee. They might as well ALL be taking place on Ork.

Is it better to succeed with an old idea or to fail with a new one? If the latter is better, "Pennies from Heaven," a year-old BBC mini-series acquired for telecast here by Channel 26 (WETA) deserves token commendation as a departure in form. Unfortunately, the program, which airs in six installments beginning tonight at 10, is also an irritating bore.

Writer Dennis Potter tells his '30sera tale of a struggling sheet music salesman and his wife by interpolating old British dance-band recordings of songs like "Blue Moon" and "Smoke Gets in Your Eyes" and having the characters interrupt their morose dialogue to lip-sync them.

The idea is to invade a neo-realist milieu with outbursts of expressionist fantasy; men even sing in women's voices. But the net result is little more than an unnecessary insult to some cherished old songs. The device merely makes an already cloying conceit indigestibly leaden.

However, though the concept collapses, at least it IS a concept. In the British TV style, the opening credits read, "Pennies from Heaven, By Dennis Potter." Instead of making Time-Life Television rich by paying it to import clucky ducks like this one, American public television should be engendering auteurist flights and follies of its own. That it does not is one of the factors that make it a stupendous flop.