It was probably an easy call that Supergirl's first Valentine's Day-themed episode of the series' CW era would end up being the most Supergirl thing ever. Whereas Arrow is comfortably settled into its niche as Teen Angst Batman Begins, The Flash lives and dies by a formula of Barry Allen reacting in boyish earnestness to DC fanservice and Legends of Tomorrow is... well, still sort of searching for its episode-to-episode "soul," Supergirl seems to be most confident in itself when allowed to structure episodes around it's cast of adult-shaped middle schoolers awkwardly (but endearingly) fumbling through romantic relationships, where the comic book crisis-of-the-week is mainly around to complicate and/or resolve the main storylines.
In that respect, at least, reimagining Superman villain-gallery mainstay Mr. Mxyzptlk (short version: he's a genie who grants his own wishes) as an interdimensional lothario who uses his powers to pursue a romance with Kara - in essentially the same way he typically deploys them to annoy her cousin - makes a lot of sense; though one imagines more traditionalist DC fans will take some time getting used to "Mixy" as a sexy bad boy as opposed to the traditional bowler-hatted imp.
Even still, the eagerness to focus on human (or, rather, "civilian identity") romantic storylines over impending action/adventure threats (Winn literally calls a "good enough for now" on his work to find Mixzy's weakness because he has a hot date) make Supergirl the most unconventionally-conventional series in the CW's DC Extended Universe stable. But students of the superheroine's long comic book history will recognize that this kind of storytelling goes back to Supergirl's earliest appearances - for better or worse.
DC's aggressive targeting of a younger female readership in the Golden and Silver Ages remains one of the great "on the other hand" conundrums in comics, with industry historians endlessly grappling with the positives of aiming for a broad audience having deep roots versus the negatives of the outdated views on gender and sexuality that permeated many such books - chiefly the way "girls' comics" opted to center love stories, romantic jealousy, competition for affection, etc. over other types of narratives. An impressive number of Supergirl stories involve villains engaging in what we'd now immediately describe as "stalking" - or worse. Case in point: "Mr. & Mrs. Mxyzptlk" isn't the first time Kara Zor-El has been confronted with a marriage proposal of the fifth-dimensional kind.
In 1962's Action Comics #291, Supergirl and the interdimensional imp appeared in the backup story "The Bride of Mr. Mxyzptlk," which is about exactly what it sounds like it's about: Mxyzptlk materializes on Earth once again and goes about the usual set of reality-bending pranks (chiefly transforming various people at the United Nations and around New York City into Bizarros), only to be challenged by Supergirl. She's unable to thwart him in the traditional way (trick him into saying his own name backwards), but he changes tactics on his own accord after developing a sudden crush on the heroine and immediately proposing marriage - which she rebuffs.
But since Mxyzptlk can do basically whatever the plot calls for him to do outside of a handful of ever-changing, seemingly-random limitations (Supergirl's new TV incarnation claims to be prevented from making people fall in love with him, inducing or thwarting a suicide, and forcing someone to drink orange juice), "rebuffing" is easier said than done. In a move that feels uncharacteristically cruel for the Silver Age Mxyzptlk, he blinks Kara's dead parents Zor-El and Alura back to life as a token of his affections - and the newly-revived parents waste no time in announcing plans to take Supergirl back from her shocked and dismayed adoptive parents, The Danvers.
Where, exactly, does the "reunited" Zor-El family intend to go? As it turns out, to meet up with Mxyzptlk at a nearby Justice of The Peace, where this incarnation of Zor-El announces he's agreed to marry his daughter off to Mxyzptlk in thanks for having been restored to life. If nothing else, you can't say that Mr. Mxyzptlk doesn't play for originality points when it comes to messing with Kryptonians (indeed, the imagination he's demonstrated in tormenting Clark Kent and company is responsible for a great deal of those "WTF Scenario!?" Superman covers the Internet has become so enamored with over the years...)
Unfortunately for Mr. Mxyzptlk, he also tends to underestimate the moral righteousness of his opponents (Pre-Crisis DC villains failing through the assumption that everyone is as amoral as they are happens much, much more often than you'd expect). He didn't just create illusory puppets to manipulate Kara; he actually willed her real parents back to life. So when it comes time to toast the impending nuptials, Zor-El he slips Mxyzptlk a truth serum and compels him to speak his inverted name aloud - banishing him back to the Fifth Dimension, canceling out all of his magic (including Zor-El and Alura themselves) and erasing all human memories of anything that had transpired but leaving Supergirl with total recall (Kryptonians in the Pre-Crisis era also had super-memory). Thus, Supergirl once again feels the pain of losing her parents.
"Mr. & Mrs. Mxzyptlk" doesn't exactly feature anything quite so tragic (the "sad" storyline of the week involves Alex learning the downer backstory of why her girlfriend Maggie hates Valentine's Day) unless one wants to count how fast Mixy's sense of pop-culture references are going to date. At one point Mon-El challenges him to a "suitor's duel" over Kara, and his creative flourish is to wisk them onto a Broadway-style stage to reenact the "pistols at dawn" scene from Hamilton. But that's probably for the best - lack of grander/darker ambition is what keeps Mxyzptlk's omnipotence from becoming annoying as a plot device. No less than Alan Moore, in his hypothetical "true ending" for the Pre-Crisis Superman in "Whatever Happened To The Man of Tomorrow?", posited a Mxyzptlk who finally decides, on a whim, to become a straight-up villain as the greatest existential threat the DC Universe could possibly face.
Will the DCU's favorite imp be back? It's difficult to say. The "important" continuing plot thread from this week's episode will probably end up being Winn's surprising (and suspiciously well-timed) Valentine's Day hookup with a beautiful alien who claims to be from Starhaven... and makes a point of mentioning that the onetime paradise-planet has fairly-recently fallen to invasion and ruin. But if he does, it'll be one more indication that Supergirl remains in no hurry to relinquish its ties to the Silver Age.
Supergirl continues Monday, February 27 with ‘Homecoming’ at 8pm on The CW.