I think respecting trans people comes with a territory of like... just because many people will pass as cis doesn't mean that it's a great idea to use their passing as a way of legitimizing how absurd transphobia is

Transphobia isn't absurd because I "look like a [cis] man," it's because transphobia is fucking ridiculous. It would be ridiculous whether or not I passed or whether I look like a "conventional man." I use myself as an example, but ultimately, passing or appearing normative should never play into whether or not transphobia is bad.

Who in sskk would you say has the higher IQ?

IQ as in, conventionally smart? Then definitely Atsushi does. I'm sorry. I've said it many times before, but I don't think Akutagawa is a particularly perceptive person. Or witty. Or intelligent in general. It's due to external factors, he never got the chance to have anything that could resemble a proper education; but it's also a matter of his nature, he's just so impulsive, and narrow-minded, and stubborn, he really has the thickest skull ever. But seriously, especially in a world full of geniuses, Akutagawa simply doesn't shine for sharpness of mind, and is way too impulsive and instincts-driven to be a person that relies on reflection or rationality. Everything that Akutagawa does is the epitome of irrational, it's one of the greatest appeals of the character.

Atsushi is smart,,,, I've talked about this also, and I think it's less sustained by canon than for Akutagawa, but I like to think he's a very observant and perceptive person whose intelligence doesn't show because he's constantly surrounded by geniuses, but still he is smart. When it comes to observations skills, I find it easy for him to have them due to his childhood of ill-treatment and abuse: as a defense mechanism, he learnt to be especially observative of people's behaviour in order to tell what sets people off and be able to prevent any escalation, I think that's a widely shared abuse survivor experience. Something among these lines is shown in chapter 51:

I also think Atsushi is a very fast learner. He's observant, and that results in him constantly absorbing other people's knowledge. He's often asking for explanations for Dazai and Ranpo's reasonings, and although I know more often than not it solves an expositive role in the storytelling picture, there's still the fact that it makes Atsushi a person who's constantly trying to understand the reality surrounding him. Atsushi is also shown to be very cool-minded and calculating in fight: from him sliding under Akutagawa and attacking him from behind successfully eluding having to face him front-off in chapter 4, to him retracting his tiger limbs to escape Rashomon's bonds in chapter 12, to the strategy he elaborated with Tanizaki (and his ability to catch up on that) to defeat Lucy in chapter 16, to his attempt to outsmart Fitzgerald in chapter 34 (that, although failed, was still driven by rational thinking nonetheless), and the list could go on. The way in fight Atsushi is shown to ponder over and implement the advices people like Dazai or Mori offered him further makes me believe he's really good at absorbing information. And Atsushi is probably book smart, too! He's compelled by reading to the point he would even risk the orphanage director's punishment just to sneak into the library and read (not explicitly supported by canon, but I can take a guess). According to the second guidebook, he spends his leisure time borrowing books from the library and studying. Overall, he really seems to be rational in all the places Akutagawa is on the contrary driven by impulses¹.

It's like… A physics law when it comes to sskk, that Akutagawa will have the most despicable trait while Atsushi has the trait that is conventionally considered the best; or at least that's as far as my characterization of them goes. Atsushi is beautiful, Akutagawa is ugly². Atsushi is polite, Akutagawa is rude. Atsushi is pure, Akutagawa is stained. Atsushi is smart, Akutagawa isn't. Atsushi is lovable, Akutagawa is destined to cause repulsion in everyone he meets³. In the end, none of this matters: they're no different where it counts, that is, Atsushi isn't any more morally just than Akutagawa is. Atsushi in not any more good than Akutagawa is (I actually suspect the contrary is true). But as far as appearances go, it's still important to portray them as opposites, because Akutagawa being unlovable and Atsushi receiving all the love Akutagawa didn't get for being his contrary - even though deep down they're the same - is almost everything their relationship is about. It's also a big part of why they act like they do towards each other: it's source of Akutagawa's bottomless envy for Atsushi; it's source of how devoted and loyal he will grow to be for him - reaching the point of giving his life for him -, because he can't see Atsushi as anything but perfection. It's source of confidence and of that certain justified hatred towards Akutagawa Atsushi feels because to his eyes Akutagawa is about the worst person to have ever walked on earth. It's source to their wish to annihilate the other as the opposite they can't exist at the same time of. It's the reason Akutagawa had to die, because he's not the good one. Overall it's also expression of Akutagawa's thematic struggle to be good and unavoidable failure at that because of the constraints of a narrative that never wanted him to be good.

But I also think they can make it work. More precisely, I think sskk can make it work when both of them can overcome and defeat the narrative dichotomy they found themselves stuck into: by recognizing that deep common ground of “we're the same” and that where it matters, in morals, neither of them is better or worse than the other. The Beast universe exemplifies that for us readers, but they don't know Beast, so they'll have to realize it by their own. About that, I think Akutagawa already caught on, because he was faster to call out the hypocrisy of Atsushi's good guy façade, and from that it's a short distance to realizing that, as much as he hates to admit it, at his core Atsushi is not that different from himself. It's taking a little more for Atsushi to realize, because it's harder to get down from that higher moral pedestal he believes himself to be on, but with his whole reevaluating Akutagawa after he stopped killing and sacrificed himself for him (and then saved him again. And then showed him how formidable of a team they are when they find a common ground.), I think he's getting there.

Tl;dr: Atsushi is smart and Akutagawa is stupid and yes it fits their personalities, but way more importantly it's consistent with the themes they carry that translate in what their relationship is like.

¹ For further reading on how Atsushi can be witty, please refer to @/gloomierdays's tags on this post. ² For further reading on how Atsushi and Akutagawa's looks can be used to reflect their characters themes, please refer to this post. ³ For further reading on how Akutagawa being not smart (as far as conventional definitions of smart go) ties to his character themes, please refer to this post.