There’s no history of Supergirl doing well in theaters. Heck, Superman’s history itself is iffy. Wonder Woman did well (the first one) because it had never been done before, and it had strong word of mouth. But for DC, its always gonna be Batman that sells. That’s just the reality.
Wonder Woman did well because Gal Gadot is beauty pagent gorgeous in a traditional way, and because the film dealt with a well-established comic book franchise in an action film genre in which acting was kinda secondary. As an actor, she’s been good in some things, and not very good in some others. She’s a classic case of an actor who needs to stay within her limits.
Her problem now is that the lefties have realized that she’s Jewish, Isreali, served in the IDF, is pro-Israel and opposes the extermination of Jews. This makes her a political pariah to the left. She’s also married with four children, so the gender benders don’t like her.
At age 41, she needs to be careful about age appropriate roles, and studios have to be careful about using her properly. There are many, many actors with limited range who have had fine careers because of careful choices and good direction.