In “Supergirl,” a melodramatic cover of Jimmy Eat World’s “The Middle” scores a climactic battle sequence, with the song’s lyrics meant to offer encouragement to the plucky heroine: “Hey, don’t write yourself off yet…” Yet the initial box office results for “Supergirl” would suggest that audiences are, in fact, writing off the film — and continuing to reject lesser-known comic book protagonists. The Warner Bros. and DC Studios adaptation, which centers on Superman’s cousin Kara Zor-El, misfired with $37.1 million in North America and $62.6 million globally in its opening weekend. Monday’s final tally was below Sunday’s estimate of $38...