TELEVISION cults are beginning to proliferate. Otherwise-normal adults tend to get soppy about the ''Golden Age'' of television drama. Sid Caesar and the ''Your Show of Shows'' gang are terribly fashionable these days. The earlier ''I Love Lucy'' series are beginning to take on the sheen of Charlie Chaplin shorts. In the popular arts, appreciation deepens with time.

But the birth of one television cult has been almost spontaneous. It involved a man named Ernie Kovacs, who in some ways was the medium's first incredible hulk. He projected a big, albeit benign, image, topped with a mass of dark hair and a formidable mustache. He usually carried a large cigar. When not in comic character, his personality was relentlessly deadpan, sometimes verging on grumpiness. But he had a talent for wacky visual comedy, for using props in the most unexpected ways. He took television and made it his personal comedy tool. Twenty years after his death in a car accident, he remains, with good reason, a genuine cult figure.

About five years ago, an Ernie Kovacs retrospective series, which had been very popular on college campuses, appeared on public television. Now, Showtime, a pay-television cable organization, is presenting a new 90-minute documentary entitled ''Ernie Kovacs: Television's Original Genius,'' a combination of sketchy but candid biography and representative clips from his shows. The program can be seen tonight at 8:30 by viewers linked to a Showtime channel.

The executive producers of the program are Edie Adams, Mr. Kovacs's performing assistant and his second wife, and John Barbour, currently with television's ''Real People.'' Mr. Barbour, who also gets producing and writing credits, interviews a number of prominent Kovacs fans, among them Steve Allen, Chevy Chase, Jack Lemmon and Robert Wagner.

Some of this material is a bit more provocative than may have been anticipated. Mr. Allen, for instance, bristles when it is suggested that his writers back in the 1950's may have been cribbing routines from the Kovacs show. But most of the chatter is in the category of standard testimonial. Mr. Chase claims to have been very influenced by Mr. Kovacs, whom he thanked posthumously when he won an Emmy Award for ''Saturday Night Live.'' Miss Adams and one of Mr. Kovacs daughters offer more personal reminiscences of a man who, despite many private problems, was evidently very special.

There is talk about his drinking and gambling habits, about his enormous debts, about his bouts of depression. But in the end, there is his comic material, which holds up incredibly well. The excerpts begin with his initiation days on small stations in Trenton and Philadelphia. Viewers were startled and delighted by what was supposed to be a cooking expert but turned out to be a strange man who liked to beat up, literally punch, heads of iceberg lettuce. That was around 1950, and Mr. Kovacs was demonstrating that audiences would watch television early in the morning. NBC soon came up with the idea for its ''Today'' show.
Moving to New York, Mr. Kovacs settled in for a decade that would see him doing everything from his own comedy format to a game show that was so off the wall that it left contestants and panelists hopelessly confused. During this time, he created his most memorable characters and routines, including Percy Dovetonsils, the esthete with the pursed-lips giggle, and the Nairobi Trio, a group of ponderously musical gorillas.

At the same time, there were the sketches with office furniture and equipment suddenly coming to rhythmic life as the song ''Jealousy'' played on the soundtrack. Or there was the Volkswagen collapsing through the street pavement as the salesman rested his hand on the hood. Or there was Eugene, who in an Emmy Award-winning half hour, didn't utter a word as he kissed statues that instantly disintegrated or read books that emitted strange sound effects.

The material survives brilliantly. Any opportunity to see it again is always appreciated, though this particular showcase has its flaws. Some of the material is pointless, most notably the inclusion of Robert Wagner, whose connections with Mr. Kovacs seem to have been minimal. And some of the material is questionable, particularly a warm closing tribute to Miss Adams. She certainly deserves praise for salvaging her late husband's tapes and paying off his debts, but the time and appropriate place would hardly seem to be a show in which she herself serves as executive producer. But, minor objections aside, there is the sight of Ernie Kovacs. That is enough to keep any cult thriving.