I have good news and/or bad news.

[triplexxx]

I don't know about you, but I see nothing out of character with the turtles all acting light-hearted; they're all capable of acting both this way and the opposite, they're all made of more than just their stereotypical personalities that some take for granted.
And it's not just that they saved the world, but also in FF, they've more freedom, they live in a future NYC where aliens are welcome and they're no longer outcasts. Besides, they still have their fights and have times to act serious.
So they rebelled in the Journal, it's not the first time they've rebelled, and in spite of all they accomplished in the previous season, they ain't perfect and they're always gonna make mistakes and probably continue to rebel every once in a while and still have growing up to do.
And I think their rebellions are kind of Splinter's fault. He kind of deprived them of a full chance to grow up, like when he wouldn't invite them to the Batte Nexus or even allow a friend of his to tell them the story of his Battle Nexus victory. He even admitted to being an overprotective father and that his sons were growing up. He made up for it by allowing them in, and then later, even gave up his place in the tournament so his sons could have a fair chance to win, 'cause had he stayed in, he would've just won again.