It seems inevitable though not encouraging that certain millions of American children will be greeting each other in public this fall by twisting their ears and chirping, "Na-noo! Na-noo!" They will have learned this knee-slapper from ABC's "Mork and Mindy," a comedy much sillier than it is funny. It premiers tonight with a special one-hour episode at 8 on Channel 7.

One of the secrets of ABC's successful, simple-minded comedies is that they supply kids with gestures and expressions that are irresistibly easy to mimic - Fonzie's "Ey" goes right to the same retention center of the infant brain that attracts commercial jingles and facilitates total recall faster than you can say "when in the course of human events," much faster.

Mork, hero of the new sit-com, is a sprightly native of the planet Ork who proves in fact too sprightly - he is dispatched, aboard a spectcraft that looks like a giant pantyhose package, to the planet earth "to learn all we can about primitive societies." The comedy evolves, though that is too delicate a word, from Mork's problems of adjustment to earthly ways. He has to remember not to sit on his face or wear his watch on his ankle or talk to plants expecting a reply.

The arbitrary site for these mechanical shenanigans is Boulder, Colo., where a nothing if not pert young woman named Mindy takes a shine to Mork and his elfin ways. He has a temperament slightly to the left of Woody Woodpecker.

"Mork and Mindy" is so poorly directed by Howard Storm, however, that costar Pam Dawber ("Mindy") is allowed to misread a punchline within the first five minutes, and a climactic sanity trial for Mork, which has the makings of a finale in the Frank Capra tradition, splatters like the egg that earlier Mork tossed into the air. And Daniel McCraven's script, constructed to accommodate ratings-insurance guest appearances by Henry Winkler ("The Fonz") and Penny Marshall ("Laverne"), is one moronic yuck after another.

What may save the show, at least for young viewers, is the interstellar performance of Robin Williams as Mork. Williams' gift for impromptu standup comedy is an awesome thing when seen live on a stage. His repertoire of voices, faces and verbal contortions is uncanny, and his energy level a wonder - he's like Henny Youngman hooked up to a nuclear reactor at full tilt, a Harpo Marx for the '70s. "Mork" has been designed to exploit but also, unfortunately, to contain and modify this lunatic sensibility - williams has to run around saying things like "no sweat off my front" and "I wouldn't harm a harf on your choley-cho-cho."

Kids will chuckle, adults will groan, and Robin Williams should think about populating a less primitive planet than this half-asteroid comedy from ABC.