The Supergirl TV show started out as a significant hit in the ratings for CBS, but the DC Comics-based TV series has cooled off in that respect since then - and though there remains good reason to think that network heads will ultimately put in an order for more adventures with Kara Zor-El/Danvers (Melissa Benoist), at the time of writing this Supergirl has not been officially renewed for a second season. Fortunately, the superhero TV show's recent crossover with The CW's own DC Comics series, The Flash, will only improve the odds of Supergirl season 2 getting a green-light.
Kara's first encounter with Gran Gustin's Barry Allen on the Supergirl season 1 episode titled 'Worlds Finest' arrived at at time when superheroes are more prone to fighting each other than immediately working together - be it on the small screen (see Daredevil vs. Punisher in Daredevil season 2) and/or, of course, the big screen with the release of Batman V Superman: Dawn of Justice in theaters. Being a different kind of superhero crossover seemed to work out for 'Worlds Finest' too, as the crossover episode resulted in Supergirl's highest ratings in several weeks.
According to the national estimates from Nielsen (per Variety), the Supergirl/The Flash crossover episode averaged a 1/7 rating/6 shares in adults 18-49 and 7.2 million viewers overall - an increase of 31% from the previous two Supergirl season 1 episodes, though not enough to beat out NBC's The Voice in its timeslot. The ratings for 'Worlds Finest' were also good by The Flash's standards, as the most recent episode in that show's second season, 'Trajectory', averaged a 1.1 rating/4 share in adults 18-49 and 3 million viewers overall. By comparison, Fox's own DC Comics-based TV show Gotham (which likewise airs on Monday evenings/nights) averaged a 1.3 rating/5 share and 3.8 million viewers overall during its episode this week, 'Prisoners'.
'Worlds Finest' similarly amassed its fair share of positive reviews, in addition to earning good ratings by Supergirl and The Flash's standards alike. Vox's review/breakdown praised the episode as being an entertaining "madcap superhero team-up caper" that even offers something for the viewers who are familiar with only The Flash or Supergirl (not both), while IGN's own review similarly praised 'Worlds Finest' for being "one of [Supergirl's] strongest episodes to date." On top of that, many a review has noted that the lighter-hearted tone of the Supergirl/The Flash crossover (an episode that includes scenes of its heroes eating ice cream and offering each other supportive life advice, among other things) was a refreshing change of pace from the tropes featured in the recent superhero crossover events - and future ones too (see Captain America: Civil War).
HitFix's review likewise praised 'Worlds Finest' as being great during the scenes where Benoist and Gustin's costumed alter egos shared the screen together, yet noted that the episode "seemed designed mainly to get [The Flash] viewers to give Supergirl a chance." Supergirl's showrunners/creators Greg Berlanti, Ali Adler and Andrew Kreisberg - most of whom also work on The CW's Arrow, The Flash, and DC's Legends of Tomorrow - indicated to TVLine that they were pleased with how 'Worlds Finest' turned out for their part, noting that they're quite open to future crossovers between Supergirl and their other DC Comics TV series. Whether or not that happens depends on Supergirl getting an official season 2 renewal, but for the time being there remains fair reason to stay hopeful that Kara's story will indeed continue for at least one more season.
Supergirl season 1 will continue with 'Myriad' on April 11th, 2016 on CBS.