Supergirl is basically like Superman 3 & 4 all over again. Corny one liners that can make everyone cringe, bad visual effect's and flat directing. Overall Supergirl is just another bad superhero flick.
Eventually, sorcery (Superman's other weakness) was eventually going to have to be brought into the series, so I can forgive that lameness, most certainly. That's not what makes this a bad film. It's a combination of probably the weakest romantic plot (not subplot, which it really should have been), a chronically unenthused (to put in nicely) Peter O'Toole (Lawrence of Arabia, Troy, The Tudors), an overabundance of non-practical effects (which technology of the time was just not ready for), nearly unbearable banter, and the flimsiest plot built off of characters being inconceivably stupid that make the film so bad.
But then, that's really the entire film.
At least Supergirl's alter-ego had more thought put into it than "adds/removes glasses" and Selena was an... Acceptable, I suppose, villain. But ultimately Supergirl sits with aforementioned Superman III and Quest for Peace as entirely missable.