It seems a little weird that a show about a bunch of aliens integrating on Earth would focus primarily on something as culturally specific as Valentine's Day, but that's what Supergirl did! The show also used the opportunity to introduce one of Super-canon's best villains: Mr Mxyzptlk. But the fifth-dimensional imp/British man wasn't the episode's best surprise guest.
It was a full on Silver Age affair with Mr Mxyzptlk stalking Kara and Mon-El trying to murder Mr Mxyzptlk and Hamilton-style duels and Parasite resurgences and a Vera Wang wedding dress.
While the Supergirl/Mon-El romance was weird -- it's still being told from Mon-El's point of view and that is sort of like telling the Buffy and Angel saga exclusively from Angel's point of view -- a lot can be forgiven. Because the show didn't just give us Mr Mxyzptlk and Parasite (and sort of Mon-El) as villains.
It also gave us a familiar face.
Why yes that is the giant ice sculpture version of Supergirl's uncle Jor-El that Superman keeps locked away in the Fortress of Solitude.
Why yes he DOES come to life, thanks to an angry Mr Mxyzptlk.
Why yes the ensuing battle between Kara and her ice giant uncle WAS everything you could hope for.
It's enough to make up for the clunky old plot of two guys trying to murder each other over a woman whose actual desires were completely ignored. Maybe next time Supergirl can do all the Silver Age goodness without all the Silver Age sexism.