Oy. First clunker of the season, imo.
First of all, if the budget allowed, the WHOLE episode should have been a more fleshed out version of the Mars storyline. Going back to Mars is a big enough event to warrant it, and when I say "more fleshed out," I mean give the TITLE CHARACTER something to do besides be a spectator for 80% of it. I liked the stuff happening in this storyline, and I like J'onn's dad, but this whole thing felt half-assed at best. Judging by them leaving out James/Winn/Lena this ep, they have no problem leaving out regulars, and Alex/Maggie could've been left out of this one just fine.
But I get it, the budget didn't allow for that to be the whole episode. They needed something easy on the budget to devote the rest of the episode to. So it should have focused on the James/Lena/Edge storyline, or even the Reign storyline, because [blackout]unlike Maggie, Reign isn't leaving in a couple of eps, so character development for her wouldn't feel like such a waste[/blackout]. Or maybe show Alex/Winn/Guardian having to deal with some threat they'd normally need Supergirl/J'onn's help with. But using that time on character development for a tertiary character (as she's not on the show because of any connection to the main character, but to a secondary character) is a waste of screentime. Especially when it just plays as an excuse for this show to soapbox again - which feels particularly contrived when you have Maggie's dad ranting about them "building a wall to keep us out," as if the writers forgot or didn't care that they've created a world where the President is in fact NOT Donald Trump, but an alien herself with very pro-immigrant views who would almost certainly not be building any walls to keep immigrants out.
And the cutting back and forth between these two completely disparate storylines was jarring as hell, imo.
So yeah, not a fan. But to end on a more positive note, as I said, I like J'onn's dad and look forward to seeing more of him. And J'onn's space car needs to become a permanent fixture on the show. Also, next week looks interesting.