IN "Johnny Guitar," a Republic western, which came to the Mayfair yesterday, Joan Crawford plays essentially the role that Van Heflin played in "Shane." She is the law-abiding squatter who stakes a claim and builds a simon-pure saloon on land that greedy Mercedes McCambridge says should be kept open for cattle range. The only big difference in the character, as plainly rewritten for her, is that now it falls in love with the ex-gunfighter, whom Sterling Hayden here plays.But this condescension to Miss Crawford and her technically recognized sex does nothing more for the picture than give it some academic aspects of romance. No more femininity comes from her than from the rugged Mr. Heflin in "Shane." For the lady, as usual, is as sexless as the lions on the public library steps and as sharp and romantically forbidding as a package of unwrapped razor blades.Too bad, because there were possibilities in this stenciled but workable plot and in the lush accumulation of performers that Republic put into the film. However, neither Miss Crawford nor director Nicholas Ray has made it any more than a flat walk-through — or occasional ride-through—of western cliches.There's the rivalry between Miss Crawford's and Miss McCambridge's gangs and then there's a sub-rivalry between Mr. Hayden and Scott Brady, Miss Crawford's bad-boy friend. There's a great deal of talk and a little shooting, and at one point Miss Crawford is almost lynched—looking in this situation like a figure in a waxworks of famous crimes.That's about all there is to it. Miss McCambridge screeches nastily and Mr. Hayden gallumps about morosely as though he'd rather play the guitar. The color is slightly awful and the Arizona scenery is only fair.Let's put it down as a fiasco. Miss Crawford went thataway.