The character of Lennie in the novel Of Mice and Men, by John Steinbeck is not necessarily meant to appear miserable. Additionally, there is not enough information in the story that would help us conclude officially that he was, indeed, miserable. In fact, if you compare Lennie to George, Crooks, and many other in the novel, he may probably prove to be the least miserable of them all precisely because he does not know any better. Yet, there are several characteristics that Lennie possesses that would make a man with average intelligence feel quite miserable. Then again, Lennie is not a man with average intelligence.
The problem with Lennie is that he cannot control himself. He is extremely big and strong for his own good. He is also taller than his peers. To top it all, he is highly emotional and loses his temper to the point of hurting, breaking, and even killing by accident. Should a man with a higher level of intelligence go through this situation, his life would be miserable for sure. It would be no different than being a type of Dr. Jekyll and Mr. Hyde, or even a type of Incredible Hulk.
Yet, Lennie only goes as far as knowing that someone will be mad at him because he did "something" wrong. He does not know what, or the consequences of what he did, but he knows that he has messed up somehow. However, it is precisely this innocence what protects Lennie from becoming miserable: Unfortunately he makes everyone around him, especially George, miserable indeed. For Lennie is a danger to society, no matter how immature or mentally challenged he may be. This is the biggest tragedy of his life.