In ACiT, Ratchet forgets about all of the things he learned in ToD (which he had already learned in the first R&C game, but I digress) and decides that risking the destruction of the universe in order to see the Lombaxes again might not be such a bad idea.
Okay, that's a little unfair. Really unfair actually. I'd like to remind you, that Ratchet throughout most of the game isn't aware of the risk that comes with using the Clock. He doesn't even know what The Great Clock actually is. The player knows, but Ratchet doesn't. Heck, there even is an in-game dialogue between Ratchet and Alister (I don't remember where), where Ratchet wonders, if you can actually safely use the Clock to travel back in time ("Of course, that's what it was built for".
That wasn't really my main point, even if it looked like it.
The point is that Ratchet goes through the EXACT SAME character arc as he did in ToD, regardless of whether or not he knew about the threat this time.
He even has a "there's more for me here than over there" moment, just like the one he had in ToD.
Good thing you didn't pick apart ITN
It's kinda hard to talk about cardboard