"Sunnyside," now at area theaters, is the least defensible of the youth gang melodramas released so far this year. The stomping and shooting episodes may be no more squalid than anticipated, but the ostensible "star," Joey Travolta, is certainly a drabber camera subject than any paying audience should be expected to tolerate.

A perfunctory account of the arbitrary events leading to a violent showdown between rival gangs in the Sunnyside neighborhood of Queens, the depraved Warlocks and relatively goody-goody Nightcrawlers, "Sunnyside" was evidently trumped up to launch the movie career of Travolta, an older brother of John. It promises to be one of those curious movies that will be dimly remembered for decisively exposing the woeful inadequacy of a new performer in the act of eagerly introducing him.

Travolta, who taught mentally disturbed kids for several years before fancying himself as a vocalist and actor, is cast as the slow-on-the-uptake leader of the Nightcrawlers. The trouble begins when the gangs form an alliance to throw a scare into some carnival operators who have supposedly been cheating the local residents. The idea is to stage a robbery with toy guns. Only the Warlocks play for keeps and shoot people with real guns.

Although Travolta mumbles his disapproval of this free improvisation to Andrew Rubin, his amusingly sneaky, unregenerate opposite number on the Warlocks, he also accepts a share of the take, which would appear to make the Nightcrawlers accessories to both armed robbery and murder. At any rate, the situation scarcely inspires rooting interest in the Nightcrawlers, a hopeless bunch of patsies, or respect for the leadership abilities of Travolta.

As a matter of fact, the nominal hero seems to dense that it's natural to derive a perverse pleasure from watching the degenerate types outwit him.Not that it's any contest. Travolta resembles a novice teacher trying to appear stern and authoritative while the class runs rings around him.

In this case the more adept, attractive and experience performers act rings around poor Travolta, a stocky, scowling human prop. Rubin, who made an impressive film debut as the upstanding eldest son of Walter Mathau in "Casey's Shadow," is almost unrecognizable as the scraggly, vicious chieftain of the Warlocks, but it's still obvious that you could build a movie or develop a career around somebody with his presence and skill. Rubin and Heshimu Cumbuka, cast as his closest aide, contrive to keep the movie from expiring completely by having a few laughs while orchestrating their conspiratorial smirks and maniacal cackles.

Joan Darling turns up in a thankless role as the widowed or abandoned mother of Travolta and his two younger brothers. Stacy Pickren makes an indifferent appearance as Travolta's girlfriend, the daughter of a neighborhood baker.

Talia Balsam, the daughter of Martin Balsam and Joyce Van Patten, generates far more interest as Pickren's kid sister, who's keen for one of the younger brothers. The secondary romance betrays unmistakable echoes from "Saturday Night Fever," including the come-on of portraying the girl as a tiresomely shameless little hot pants. Nevertheless, Balsam appears to have a spark that reflects something more invigorating and authentic than her character's tennybopper libido. CAPTION: Picture, Joey Travolta, who portrays Nick Martin in "Sunnyside," and his movie companions.