I've known kids like Clem before. They're few and far between, but they're there.
She was mature beyond her years, but Telltale succeeded in the fact that she was still a child. She still had an air of childlike innocence to her, she still failed to grasp the full gravity of some situations, she was still a little naive, couldn't always wrap her head around and accept what you told her, but she wasn't what most people do when writing kids and make them complete clueless morons - which some are, but some are also like Clem. She picked up on things, she knew more than she let on, etc etc.
Some might say she was unrealistic, but I thought she was the most realistic child character to ever come around in a game.