Humans are sentient animals. All the "animals" in FF9 are also sentient animals. I see no reason why a game should be less serious just because it contains other sentient species in addition to humans.
The method of saving is equally absurd in both games. I mean, a ball of "energy" that magically let you live forever. How realistic is that? Furthermore, there are sentient races other than humans in every Final Fantasy game. How is a humanoid rat worse than a moomba or a shumi?
How "serious" a game is is only dependent on the level of realism if you have a total lack of imagination. I'm not saying you're not allowed to dislike the game, in fact, I actually dislike it myself, but your reasons are in my opinion very shallow.