"In past decades, a superhero movie with a female title character was considered box-office kryptonite in most Hollywood circles. That view was only reinforced by radioactive failures like Supergirl (1984), Tank Girl (1994), Catwoman (2004) and Elektra (2005)."
To be fair, Supergirl was a terrible movie. I had heard that Christopher Reeve was to reprise his role as Superman but shortly before shooting began he withdrew causing the script to be rewritten pretty much as the movie was being shot.
A version of that rumor, #15 at: https://screenrant.com/supergirl-worst-movie-shocking-trivia-facts/
From that same site is this tidbit:
"Supergirl became the lowest-grossing installment of the “Super” franchise with only a $14.3 million domestic take. Superman IV: The Quest for Peace did slightly better in the summer of 1987 with a $15.7 million take in the United States."
I do recall seeing Supergirl with the then wife on opening Sunday in Indianapolis, admittedly, movies on Sundays have always had low attendance, but there were only two other people in the audience for this showing.
Which, btw, tied with our experience with Grease II.
And, I remain sufficiently geeky to own a copy of the newly released "Warner Archive" Supergirl BluRay. (Grease II didn't make the cut)
I always wondered what happened with III and IV with the Christopher Reeve movies. They had lots of comic book villains to choose from, and they didn’t spare with the special effects; no chance of bringing Brainiac, Bizarro or even Darkseid into the theaters back then? (The way they did III, they could have thrown Mxyzptlk in there too.)