Warner Bros. claims that a "positive reaction" from critics, indeed a "tremendous response from the press," forced ABC to bring back "Off the Rack," a new sitcom set in the garment trade that got a trial airing in December. The response wasn't all that tremendous, really, and network executives pay no attention to critics, except to hate them. But as one of three comedies ABC tested on the air that month, "Off the Rack" was certainly the least awful.

One might even go farther in praise. It was . . . Not That Bad!

And so the show returns for a regular run beginning tonight at 9:30 on Channel 7, part of an ABC Friday night salvage operation. "Rack" has possibilities, and the writers and producers may actually investigate one or two of them. The biggest plus is the casting: growly old Care Bear Ed Asner as clothing manufacturer Sam Waltman and wily Eileen Brennan as Kate Halloran, the widow of his partner. In the pilot episode, much to Sam's displeasure, Kate decided to succeed her husband in the firm. The premiere picks up six months into this uneasy alliance.

Many the TV comedy that has been built around long-running spats. The Brennan-Asner match-up is in a venerable comedy tradition, and it's not unlikely that even Hepburn and Tracy would be amused by it. Or by them, anyway. Asner, big and fat again, seems to have turned perfectly round. He's all scowl, like the villain on "The Smurfs." And when Brennan issues a rejoinder, it stays issued. Or rejoined. There's plenty of crackle in her snap.

Looking over a designer's sketches that she hopes will be terrible, Brennan manages to say "Oh, just look at this! I mean, it's positively fabulous" with a mock-lugubrious, Middle-Earthy groan. The writers have made Sam disagreeable to perhaps an excessive degree, but Asner warms him up a little. His best line is near the end of the show: "Kate, you said it best when you quoted me."

In the premiere, Asner's son Matthew is seen briefly in the role of a punk designer. New to the cast are Cory "Bumper" Yothers as Kate's little boy and Dennis Haysbert, who keeps his dignity in the role of Cletus Maxwell, the only employe who can intimidate the bellowing Sam. Cletus is not only tall but black; the role is thus conceived in terms that are racially demeaning. Haysbert makes the best of a bad situation.

Computers may one day be able to write formula sitcom television scripts as well as humans now do. The thing that no computer will ever be able to do is tear off a retort the way Eileen Brennan does. When you watch "Off the Rack," you are aware that Brennan and Asner have located whatever particle of invention exists in the script and given it sizzle. "Off the Rack" is probably the best new comedy ABC has come up with this year. Too bad that's such lowly praise. 'Mr. Belvedere'

"Mr. Belvedere" is the sort of thing people have in mind when they say they hate situation comedies. A mechanical throwback to times even dumber than our own, the ABC comedy, premiering at 8:30 tonight on Channel 7, stars the underrated and ill-used Christopher Hewett, a character actor with sardonic grandeur, in the role created by Clifton Webb for the 1948 film "Sitting Pretty": that of an effete and officious, but somehow endearing, male housekeeper.

Following also in the footsteps of Sebastian Cabot ("Family Affair") is a footstep up for Hewett, at least from his last insulting assignment, which was to succeed Herve Villechaize as chief plane spotter on "Fantasy Island," but the level of comedy writing is low and the imagined domestic frictions paltry.

In the original film, Mr. Belvedere brought order to an anarchic household that had gone to bedlam in part because Momsy had decided to pursue a career. In 1985, you would not want to suggest, even in a comedy, that working mothers lead to messy homes. So the writers blame the problems more or less on the father, a dithering fathead played by beer commercial habitue' and former baseball un-great Bob Uecker. Justice, or at least modern thinking, is served.

Mom, a law student by day, is played by the undistinguished (but more distinguished than Uecker) Ilene Graff. The one actor with whom Hewett gets some sort of comic chemistry going is the youngest one on the premises, Brice Beckham as Wesley. There's a smidgen of sympatico there, and Beckham has one of those dauntingly slick kiddy deliveries, but this isn't enough to save the show -- any more than is Leon Redbone's most agreeable rendition of the title tune. Only once or twice does "Mr. Belvedere" ring the bell.