You'd think but Vivi/Luffy weren't forced to travel 20 years in the future and have a younger sibling who is now older than they are. Not only that Momo is technically burdened with being too young to lead. It's a hefty part of his character and this arc.
CONSIDERING Shinobu's power works on living things, how does "maturing" Momo's personality would work?
Is Oda gonna try and play God saying to the world that whatever personality Momo end up with is what a "truly mature person" should be?
Who does Oda think he is to declare such a thing?
That's why I think that, EVEN IF Shinobu turns Momo into an "adult", Momo will have the exactly same mentality he currently has.
So I ask, what good does it even do?
I see 3 options.
A- Oda decides that he knows more than anyone that has ever lived and he alone can tell what a mature person is, by making Momo become adult not only physically, but also mentally.
B- Momo turns into a man child. Which will make him even more pathetic.
C- It's a bait and no way Shinobu's powers work that way.
For the sake of salvaging whatever "quality" One Piece still has, I'm choosing to believe in option C. Not for the sake of "trying to solve whatever Oda's planning", but just for the sake that I see no scenario with the currently knowledge that we have, that an "Adult Momo" is a good solution for anything aside from Oda trying to copy the "Adult Gon" thing, and doing an exceptionally crappy job in doing so.