Twenty years ago this month, "Mary Tyler Moore" premiered on CBS. What has happened to the art of the sitcom since then? It appears to have lost ground rather than gained it. Many of this year's crop suggest regression in content as well as style.

Three new specimens float to the surface tonight, all of them about goofy guys. Two of the shows are actually pretty funny, if hardly innovative -- "Fresh Prince of Bel Air" on NBC and "Uncle Buck" on CBS. In addition there's another, lowlier CBS entry, "Lenny."

Unfortunately, the two that are funny go up against each other at 8 o'clock: "Fresh Prince" on Channel 4, "Uncle Buck" on Channel 9.

Will Smith, the immensely magnetic rap singer known as the Fresh Prince, slides and glides confidently into his new tailor-made sitcom, but the tailoring is shabby. He's supposed to be a street kid from West Philadelphia who goes to live with his Uncle and Auntie Banks in plush and cushy Bel Air, not far from the Reagans (who do not appear).

The series, which bumbling, absent-minded old NBC sometimes calls "The Fresh Prince of Bel Air," obviously owes something to gimmicky hits of the past like "The Beverly Hillbillies." In fact, the way its star and title character are treated, the show also owes something to "ALF" and "Mork and Mindy."

Like ALF and Mork, Smith plays a wisecracking Fresh Fish-out-of-water, a lovable alien riding out culture clashes. "I used to fence," says a snooty guest at a party. "Really," says Fresh Prince. "How much do you think you could get me for that stereo?"

The writing, by Susan and Andy Borowitz, is slack, especially in the anachronistic way the script depicts the rich. The Bankses act more like a wealthy family out of "Masterpiece Theatre" than 1990 Southern California. They even have a hoity-toity butler named Jeffrey who refers to Smith as "Master William."

Perhaps the writers, director Debbie Allen and executive producer Quincy Jones are afraid to show America how rich people in L.A. really live. As for the new arrival's "outrageous" behavior, that consists mainly of using slang expressions, playing a tune on the drinking glasses at the dinner table and wearing a funky tux to the party.

Gracious goodness sakes alive, what next???

The cast is pretty dull too, except of course for the star, plus Alfonso Ribeiro as cousin Carlton, a square young yuppie-to-be who, asked to name his heroes, ends the list by saying, "And Bryant Gumbel. He's darn good." It's a darn good line reading, not spoiled by the fact that it's already been seen in publicity clips about 150 times.

Fortunately for Smith, he's such an utterly enigmatic charmer that he easily surmounts the material; indeed, his balletic swagger seems to be telling us he knows that he, and we, are above it. Under the circumstances, he tries to have and give as good a time as he can.

"I'm a joker. I play around. I have fun," he says convincingly.

As he did in his music videos "Parents Just Don't Understand" and "I Think I Can Beat Mike Tyson," Smith cavorts with panache and -- rare for a pop star -- a healthy sense of self-mockery. It's not just easy to like him; it's almost impossible not to.

"Uncle Buck" doesn't have quite such a commanding presence at the center, but the script, by executive producer Tim O'Donnell, is a far more accomplished piece of work than "Fresh Prince," and the pilot episode crashes out of the starting gate at a wild roar, thanks largely to the zip imposed by director John Tracy.

The title character, derived from the John Candy movie of the same name, is a pudgy, crude, cigar-smoking vulgarian left in custody of three kids after his brother and sister-in-law die in an auto accident. As the series begins, we're five months into this experiment in terror, and, wouldn't you know, Uncle Buck has his hands full.

Little Maizy (Sarah Martineck) uses expressions like "freckle butt" and "booger ball," not to mention "you suck," as heavily documented already. Equally little Miles (Jacob Gelman) says he was beaten up by the school bully -- who turns out to be a girl named Patricia.

And not-so-little Tia (Dah-Ve Chodan), 16, has a habit of dating the wrong men, apparently in bulk.

Into this near bedlam, even as Buck is entertaining lowlife pals at his weekly poker game, strides the kids' grandma, Mrs. Hogoboom, who immediately threatens to wrest them away from the slobs' paradise in which they appear to be living.

Kevin Meaney, inheriting the Uncle Buck role, handles it as smoothly as frogs handle flies; he's full o' fun, from the first shot of him in his Cubs cap making sandwiches and dancing to "Great Balls of Fire." The two little kids are agreeably cute and spunky.

Lu Leonard is solidly funny too, as Ms. Crappier, the school vice principal whose only smile is on a button attached to her lapel. But the high point is the arrival of Audrey Meadows as Grandma, marching into the show with the unmistakable authority of an unimpeachable pro.

The fact that Meaney has a slightly Gleasonesque persona gives his exchanges with Meadows, so long ago Alice Kramden, a nostalgic oomph. Cheering memories of "The Honeymooners" hover over the proceedings -- the icing on a fast-frozen devil's-food cake.

For the record, small improvements were made on the "Uncle Buck" pilot after it was shown to critics in early summer. The character of an insurance agent named Doreen (Jill Jacobson) has been softened a little from its previous borderline and inappropriate nymphomania. She no longer suggests to Buck, "I was thinking along the lines of doing it under the table."

CBS, meanwhile, imagines it has a yet another working-class hero in "Lenny," the sitcom getting a preview at 8:30 tonight on Channel 9 before moving to a Wednesday-at-8 slot on Sept. 19. Stand-up comic Lenny Clarke plays stand-up fella Lenny Callahan, a blue-collar straight-arrow with a wife, three children and a lot of boring headaches.

Among them is a cantankerous father played by Eugene Roche a la Archie Bunker, and a brother, Eddie, played by Peter Dobson and referred to by Lenny as "a slimeball," "a bum," "the deadbeat of the decade" and "a low-life parasitic leach." What an unpleasant bunch of people.

Lenny's embattled wife, played by Lee Garlington, may be the most interesting character around but gets little screen time. Also making a big impression in a hurry is Martha Jane as a neighbor named Mrs. Luby, who walks into the Callahan house to announce, "The Lubys have always been large people. We're descended from Goths."

In the pilot, written by Don Reo and directed by Terry Hughes, Lenny takes out a loan to help pay for dad's hip operation. He has to swallow his pride because he's never borrowed money before. Dad has to swallow his pride and admit he needed help. Any more pride swallowing and everybody might choke, including the poor viewer.

Dialogue runs to such unfunny excesses as "Pardon me, but this is a load of crap that wouldn't fit into a Winnebago." We've come a long way from "Mary Tyler Moore," haven't we? Oh, Mister Grant!

"Lenny" and its star are supposed to be the CBS answer to ABC's "Roseanne" and its star. But Lenny Clarke doesn't have what it takes to make Lenny Callahan a galvanizing figure. Clarke is sort of like character actor Danny Aiello without the -- well, without the Danny Aiello. The show is loud and loutish.

Cranky old pops is ridiculed a couple of times for sitting on his couch and watching the Weather Channel. When you get right down to it, that may be a preferable alternative to watching "Lenny."