I just feel like the reasoning that he acts like a stereotypical James Vega bro, to counter his Qunari appearance and allow him to infiltrate foreign societies easier, is really just headcanon. It makes sense but it wasn't really presented in a way that made that intention apparent to us. Especially since he made a big deal to inform the inquisitor anyhow that he is a spy. This outward personality as a facade should've been remarked on in-game at least, by someone who's met Qunari before. Perhaps Krem could've said something, or Cullen could've had a line or two about how the Qunari he's met in Kirkwall seem different than Iron Bull.
Without commenting on it in game, it really just looks like they made him a bro because he's big. Which is really dumb. The Qunari are much more than that.
I might have phrased it oddly, English isn't my native language, but I didn't mean his personality was only a facade. In the beginning, when he was just starting out, it might have been but by the time DAI comes that personlity has become real.
What I meant by bringing up the whole Hissrad role was to bring up that he's more flexible than the average Qunari because his role demands it. It's not a common role, or one that everyone is suited to but it's still necessary to the Qun. Sten and the Arishok are much stiffer and inflexible about the Qun, there is no compromise because a compromise would be a "lie". Any attempt of any of them to act as a spy would be doomed to failure because of that rigidity. The Arishok's best attempt at "compromise" was Kirkwall - and we all know how well that turned out.
So Qunari Hissrad being less direct in order to fulfil their roles isn't surprising. By drinking and being a boisterous blood-knight Iron Bull seems more like a Tal-Vashot -which is what he was pretending to be- and more appealing to the nobles that hired him because the Andrastian nobility fear and hate the Qunari and their spreading influence. That he found he genuinely liked drinking, fighting and the camaraderie with his mercenary group - especially after how his previous mission turned out. Seheron was apparently such a difficult place that the recommended time for a Ben-Hassrath there was at maximum 2 year and he spent way more than that (can't remember exactly how long but 6-10 year) and submitted himself for re-education. He was having trouble with the Qun before DAI but he's clearly loyal to the Qun (pre-companion quest) even if he's not in your face about it.
So, instead of seeing Iron Bulls character as going against previous Qunari lore, I see him as expanding it. After all, aside from Tallis, the Iron Bull is the first Ben-Hassrath we can talk to in-depth. You can't expect to know an entire society by just meeting one branch of it.
Also, acting as a bro, I take that to mean not so sharp (among other things), is far from reality. I really believe the above was intended by the devs but you might be right that it's just headcanon but Iron Bull is clearly meant to be intelligent even in-game. The guy has a mental game of chess with Solas (without a chess board) - stupid he is not.
I hope that made my point clearer.