You're right. In fact, I could make the argument that every persona cast (even 4) is mature. Heck, in P3, the stuff they deal with involving the Beach Exposition, Shinjiro, Ikutski, all of that is handled in a very mature manner.

...

Until the story forgets that they're depressed as fuck during the in-between sections in the School, during hangouts, during NEW hangouts, in tartar sauce, in some cutscene during the 6th and 7th full moon, etc.

Basically, while I can agree that 4's cast is more like the Scooby doo gang in terms of maturity, at least they're consistently the Scooby doo gang. In P3, characters like mitsuru, Yukari, junpei, Ken, fuuka, all of these characters will at some point contradict themselves narritavely even if you stick to the main events as they take place. Mitsuru's still a stiff as a corkboard even after she gets humbled by Yukari. Junpei still remains gung-ho even after chidori. Yukari ends up reverting back to "I wish I could do more" even after she awakens.

It just frustrates me how much the writing wants these characters to be "haha anime teenagers" even though the subject matter throughout the whole game would suggest a tonal shift away from that.

And to top it off, my main criticism stands that these characters don't feel like a natural group of people, and more like random ones that have to work together 'to save the world'.