I believe the creeper is a complex analogy for the folly of any human endevour.
It symbolizes fate in it's purest form, by telling you, the player:
No matter what you may or may not accomplish, I will be there to take it all away from you. No amount of walls can stop me, no matter how superior you fancy yourself to be, I will wait.
In a way the Creeper resembles Death, it is the Minecraft Version of the Grim Reaper.
These to facts are of course reflected in the Creepers design and it's sound. The simplistic form reminds of the plain nature of Death&Fate. They don't need to be complex, their unstoppable simplicity is what makes them horrible.
Then there is the sound, this plain ssssh, like a soothing sound made by the insane murderer that just leaped out of the dark to strangle you. It is supposed to put us at ease, because we can not stop the inevitable and all struggle is ultimately futile.