I don't really get the idea that Sora was dumbed down in DDD. He was pretty happy but this was also the first adventure he ever got to go in where he was on it pretty much just to support someone, not because he needed to go. He doesn't need to worry about Kairi or Riku, his home is back, the worlds are back, he's worked out a lot of his issues. It's the most relaxed adventure he has ever gotten to have and that's reflected in his being a fair bit less brooding.
At the same time he shows he is way more insightful and intelligent than he was in the past. He starts picking up on the Nobodies having hearts in Prankster's Paradise, and once he saw Roxas's memories he knew the truth immediately. Compare that to KH2 where despite his entire adventure and all he experienced he shouts at Xemnas's face an insistent belief that Nobodies don't have hearts and feel nothing.
He completely recontextualized what the keyblade meant to him and his place in the world (partly because the narrative required it since he was no longer THE keyblade master after BBS). In KH1 it's just a weapon, in KH2 it was a weapon that as long as its in his possession means he must keep fighting. But in DDD it's more than that. It's a power he has because of his friends, and so it's not a power to misuse for his sake but instead something he needs to always use for the sake of others. To him the keyblade is the chance to make people happy and that is something he could never give up. You can see some real maturity in him as well in regards to how he interacts with the residents. Like La Cite Des Cloches he doesn't learn any meaningful lessons because he already knows the lessons that the characters in that world are still trying to learn, and he instead tries to teach them and he does that in lots of worlds. Before Sora was the one learning but DDD Sora is almost always in the role of the educator. Then there was the Rinzler stuff and how he was able to use his heart and the keyblade together to literally rewrite Tron's heart setting him free with the power of their bond.
Sure he can be goofy and forgetful at times, but he's always been like that like hell he forgot he turned into a heartless which is something far more meaningful than one of the many pieces of Yensid's droning lecture info dump that he had thrust on him all at once, most players don't even follow what is going on most of the time in DDD so it's extra unfair to judge Sora for not fully grasping it either. He also was reckless knowingly running into a trap but this is the kid who in KH2 after barely escaping a bunch of Nobodies in Twilight Town thought it was a good idea to walk into the Underworld and demand Hades give Hercules a break.
Yeah, Sora has never been this pinnacle for maturity. KH1 he's different because (other than it being the very first game of course) Riku and Kairi have gone missing, his world's gone poof, and now he's got this weird mystical weapon with a ton of responsibilities that come with it. Chain of Memories is Sora being manipulated, and anybody would feel anger in that kind of situation.
KH2 Sora is more laid-back than he was in KH1 because he's experienced enough with the Keyblade, and his real main goal was to find Riku and go back home to the islands. It wasn't until Yen Sid told SDG that "Oh yeah, Organization XIII is trying to do bad things... And if you're lucky, you might find Riku on the way there, since everything's connected!" Sora even thought about doing anything else. Plus Kairi was home, so he at least had one less worry on his mind. Sora was plenty goofy in KH2, but when it came down to it, he was serious.
DDD is the same way, and you've explained it perfectly. Sora didn't give a shit about the Mark of Mastery exam, far as he was concerned, it was a stress-free adventure with no real stakes involved. He only really did it cause he was told to, and because Riku really needed it more than anyone else did, so he's just going in with the intent to support his best buddy. It wasn't until shit really hit the fan when Sora took the situation every bit as serious as it needed to be. Plus yeah, like you mentioned. Sora redefined what it meant for him to be a Keyblade wielder, and if he was "stupid", he would've never arrived at that conclusion. The series hammers it in that he's not "the chosen one", but in his words, he says himself that he doesn't care. He's part of something bigger, the people it did choose.
KH3, I think he was plenty serious for the most part. He still had his fun like he usually does with Donald, Goofy and whoever else is with him in said Disney worlds, but he was a lot more mature this time. He's cocky and reckless, a bit forgetful when it comes to maintaining the World Order, and the main cast for whatever reason like to hammer it in that Sora "still needs work", but he's incredibly more mature when you see the moments in KH3 that really require him to be introspective. Even the end of KH3 when he's talking to Xehanort, KH2 Sora or even KH1 Sora wouldn't say something like that.
Kid's doing just fine as he is. I don't want him to be so serious that he's basically Riku, because we have Riku for that. And even though all of that, it's still Sora that has to go clean up after everyone anyway, so it's not like his maturity even really matters in the long run.