I think in the comics Supergirl is actually stronger than Superman since her ship went around the sun so she absorbed a whole lotta energy or something.
First of all, " in the comics " doesn't mean much, as many stories are retconned or the situations temporary.
But even then, it's not about Kara being stronger than Superman, I'm not against that by default.
It's about Superman being a worse version of Kara in every way possible.
There are two problems here.
1) Supergirl was in awe of her cousin all this time and somehow is now leagues above him for no reason. Even she was surprised it was that easy to beat him.
She was holding back and he was fighting all out to save earth. Sure.
2) Usually in stories where Kara is stronger, Superman is the voice of reason, using his experience to guide her, and still being a worthy hero with incredible stamina that doesn't give up. I'm sure there are some comics where maybe he's worse all the way but they obviously didn't stay around for long. Now they usually have complementary attributes, where both can shine.
Here he is an idiot who tries to be patronizing even though he sucks and Kara is faster, stronger, smarter, and more grounded than he will "ever be".
The fact that the writers had to spell out 10 times this fact shows how poor this situation was conceived.
They didn't try to have Superman be Superman and Kara being above that in specific ways, they shit on Superman to make Kara look brighter.
In no dimension is it fine to have Superman's legacy and strength as a hero trivialized like they did, especially not when it's not a story about him where he won't get any growth from it.
This isn't a "Kara is better" issue, it's how they had to throw Superman under a bus for the sake of making Kara look more impressive.
It's lazy writing.