The Terminator is likeable in its thumpy way precisely because it is low-budget, tacky and transparent. Supergirl, another space-help-wanted cartoon, suffers from lowered expectations as well as tired blood. Superman through superman III have just about exhausted whatever interest there was in Kryptonic culture, and when Supergirl (Helen Slater) bumbles to Earth on a teen-age lark, there is nothing very original for her to do except show her postpubescent cleavage (funny, Christopher Reeve's spandex bodystocking went all the way to the neck). Her journey would be a total loss if it were not for the appearance of Faye Dunaway, who plays a carnival fortuneteller and Supergirl's nemesis. In a sense it is too bad that Dunaway has been consigned to rococo roles and gothic pantomimes, but she seems to take enormous please in the parts. Nobody struts and stares better; those eyes of Laura Mars can melt kryptonite and those Joan Crawford shoulders can pierce the Teen of Steel. Unfortunately, the plot has Dunaway and Slater fighting like two alley cats scratching over a tom, and that's a far cry from the noble mission of the supermale. Jeannot Szwarc, who directed, is now at work on Santa Claus, another supermovie due out next Christmas, and one can only hope that he doesn't run another myth into the ground.