Aslan: More Messy Than Messiah
Narnians don't directly call Aslan a god, so he comes off as just a powerful, magical figure like Gandalf. People can ask him for help. He may or may not give it to them. But they don't bill him as creator of the universe. He's playing along with certain rules that he has to work with, not like he's the Maker of all rules and laws of physics.
Like when he sacrifices his life, knowing that he'll come back a few days later. Actually this is a lot like that other guy who comes back after three days. If he's really confident that it'll happen, then why is it a big deal?
In Prince Caspian, Lucy asks Aslan one of those questions skeptics ask in our world: "Why didn't you come to save us like last time?" If you're a good guy and you have the power to help people, why do you go away sometimes?* Why do you only appear to some people? Why did we have to wait for half the army of good guys to get killed in battle before you'd come out of hiding and use your powers to help? You only do good if a little girl rides through the woods a long way and asks you to come out? You won't show yourself unless she has the sincerest pumpkin patch?
Aslan's response to Lucy is, "Things never happen the same way twice." Not a very satisfying answer. It sounds like he's saying that he doesn't make the rules. He does the best that he can within his limitations. That makes some sense if he's a magical creature in a world created by someone else, but not if he's the one God who created everything and who can do anything.
At least we don't have to hear Aslan talking down to us like, "You humans couldn't understand how the universe works, so you won't be able to understand why I do the things I do. I know things that you don't and I can't explain it to you. You have to take my word for it that I'm good." That wouldn't fly in a fairy tale like this. I'm not using "fairy tale" as an insult, but it's how Lewis classified the story.
I suppose "Things never happen the same way twice" is intended as some kind of explanation. It just doesn't work very well.
There's at least one other way that the stories about Aslan are a bad analogy for Christ. The child heroes meet Aslan in person only a few days after first hearing about him. They see him, touch him, hear him, maybe smell him. He works magic and they see the results for themselves. This is not like believing in Aslan's magic based only on reading a book of 1500-2000 year old stories, or based on the testimony and peer pressure of people who say they believe in him or felt his presence. Narnian believers have a naturalistic world view. They rely on their senses. If there are stories about other magical creatures or gods in Narnia, they might exercise a healthy skepticism until they have personal experience or some evidence that the stories are true. That's not how Christians generally ask us to form our beliefs about the Bible.
It's the same kind of failure when Horton Hears a Who is supposed to convince us to have faith in unlikely things, but in the end, it shows everyone directly experiencing the unbelievable events. None of the characters rely on faith. They're skeptical, materialist, naturalist, secular Whovians or secular Narnianists.
[I haven't read the books yet, just watched the first three films. So this critique only applies to the movies. Maybe the books are better. But I've read some C.S. Lewis including The Great Divorce, and I doubt the Narnia books explain how a good creator would let bad things happen.]
* Same question applies to Superman Returns, The Dark Knight Rises, and Luke in The Force Awakens. But those are fallible humans, not perfect gods.