Yeah I agree. Rodimus' maturity was really achieved by his contact with the Cosmic MacGuffin that is the Matrix.
Odie: Hot Rod's journey in TFTM didn't have any lasting effect on the character in making him actually mature. At best he may have had 'moments of enlightenment', but did those moments actually mature him? Because if they did then we should see the character taking forward steps as he develops. The problem I see is that, as jimoinj said, there's no real growing up at all. Whenever Rodimus Prime loses possession of the Cosmic MacGuffin, he reverts back to being Hot Rod again -- and not just in a physical sense, but his persona becomes Hot Rod too; i.e. he becomes more relatively immature than he was before.
When a character matures, they're not meant to go backwards unless there's some kind of experience that makes them take that path (e.g. Darth Vader reverting back to being Anakin Skywalker again). I like how Paulbot described the MacGuffin as like a Power Up mushroom in Super Mario.
You compare Rodimus say with Frodo Baggins. His contact with the One Ring and his journey to deliver the Ring to Mount Doom in Mordor drastically changed him as a character. But once the Ring was destroyed, Frodo did not revert back to being his happy go-lucky cheery old self, not even after he returned to the Shire. He
tried to get his old life back living in Bag End, but it just didn't work -- thus the Elves took him and Bilbo to the Grey Havens to look after them... for they had irreversibly changed.