Some middle schools have a very supportive and accepting atmosphere where musical talent is celebrated. In these environments, being a singer could be considered very cool and popular.
In other schools, there might be more pressure to conform, and singing could be seen as less "cool" by some peers.
Ultimately, what's important is that *you* enjoy singing. If you love to sing, then do it! Don't let others' opinions dictate your passions. Your confidence and enjoyment will shine through more than any perceived "coolness factor."