Would that not make both of them interesting tho? The argument was that she was going against the grain, but both of them are going against the grain. If neither of them are behaving the way you'd expect them to. Why are so many people saying she is boring, but the other is not. If both of them are going against the grain, and not behaving the way you'd expect. She has been good at anything she has tried except for this skating event. Is it boring to be naturally gifted at things? If the MC was naturally gifted at everything I would be willing to bet 90% of the people complaining about her being gifted would not complain about the same for him. It's a double standard he would be labeled as someone talented and praised for it, but she is labeled as someone boring, and Mary Sue.
So - you're conflating multiple things together that I don't think actually have any connection.
I say that Hebikawa and Kusunoki are "not acting like you'd expect", because neither of them are acting like they are in a romcom series. Kusunoki's entire character arc & goal was "a successful high school debut and making friends", and Hebikawa's is "prove that Kusunoki is actually faking it all and is really a dark & sadistic person".
Neither of those line up with what Otobe & Keisuke are doing, which is "getting their crush to go out with them", i.e. a much more standard romcom formula.
But that "going against the grain"
also doesn't imply that Kusunoki isn't boring, or that Hebikawa is; the reason I personally find Kusunoki "boring" as a character is because she was shown as this timid, uncertain, wallflower of a girl who would be a background character anywhere else, but the moment she tries to do anything - she's suddenly perfect and garners instant adoration and attention from everyone around her. The only thing she needed to "solve" her character arc was the almost-literal handholding of someone else to Do The Thing, and suddenly it was solved because she's just naturally talented, but insecure.
That in and of itself is fairly standard fare, and her overall character growth line is very flat across the plot, compared to the other three. But beyond that - Hebikawa believes that Kusunoki is being fake and hiding a "Dark Side", but she's not.
She really is just a talented, nice, polite, demure, beautiful girl who wins the hearts of nearly everyone she talks to and succeeds at whatever she does the moment she puts in any effort. There's no payoff; it's just a given she'll "be the best" and be popular as a result, because it was always there - she was just insecure. As character write-ups go, that's pretty standard, and thus
comparatively boring when put alongside the rest of the cast.
Hebikawa, by contrast, isn't shown to be nearly as gifted, and has the added complexity of
actually wearing masks, like she accuses Kusunoki of doing. She pretends to be one way, but is really another, and we only see that "truth" when she's alone with Keisuke and allows the mask to slip. She's also starting from a much less-liked position compared to the others, meaning the overall trajectory of her character arc toward an improved, "happy ending" is much greater, and she has more room to grow and change than the others do. Hence, more
narratively interesting.
To your last point - trying to say Keisuke wouldn't get the same flak as Kusunoki if he were perfect is a false dichotomy. For one, I'd argue that
Kusunoki is the "Main Character", and that Keiske is simply the lens character through which we experience the brunt of the story.
He's the tie-in that sees the rest of the cast come together, but it's all centered around Kusunoki, as she's the pivotal point that
all of the rest of the stories of the cast revolve around. Otobe is her best friend and literally calls her "my lady"; Keisuke fell in love with her; Hebikawa is her narrative foil and antagonist.
The only way it would make sense to make Keisuke "as capable" as Kusunoki, is if you inverted the entire cast's gender and roles within the story, and make Kusunoki the "lens character" who has three guys around her - one who she pines for, one who wants to ruin that romance target, and a best friend who falls for Kusunoki who inevitably loses.
In that context - yes, Keisuke being naturally gifted at everything
would draw the same criticism, because he'd clearly just be a perfect specimen of a himbo who's smart, athletic, charming, but insecure and lacking confidence--just like Kusunoki. But at that point, the entire style of manga would be different (much more
Josei, most likely), and that added subtextual difference would probably throw any realistic comparison out the window.
All that is to say - I honestly agree with your overall original sentiment about the flak toward Kusunoki, and the fandom toward Hebikawa, and especially about the issues surrounding people simply leaving hot takes about characters without considering the context of the story and their roles therein. But there's multiple layers of discourse happening simultaneously, and they're not all really comparable in terms of what relates to what--some people are effectively shitposting, some people are actually discussing the narrative and the characters with varying degrees of literary competency, and some are just arguing about whether or not one of the first two groups' points are valid.
The issue arises when you try and equate different "types" of discourse and claim they stand for something they don't in the larger context of how the readerbase receives and responds to the series being discussed.