Perhaps because he is a lamb, his actions in the story are, quite literally, those of a troublemaker. Even compared to the other sheep, he is almost unrivaled in this regard. He is constantly causing accidents, and Bitzer and the other sheep always have a hard time cleaning up after him.
There was even an episode where the sheep were targeting the farmer’s picnic lunch with his girlfriend. Thanks to Timmy’s cuteness, they were able to win their trust, make them let their guard down, and then eat everything.
His main trademarks are his pacifier and teddy bear, though sometimes he is shown without one or even both. However, if even one of them is taken away, he often cries. As the series progresses, though, he grows up and almost never cries anymore.