No, braces will not ruin your reputation in middle school. While some kids might tease others for any perceived difference, having braces is extremely common. Most people understand that braces are for improving dental health, and it's a temporary thing. In fact, many people consider braces to be a sign that someone cares about their appearance and overall health.
The impact of braces on your reputation depends far more on your overall personality and how you interact with others than on the braces themselves. Being confident and kind is far more important to your reputation than having straight teeth at that age.