The more you sing, the better you will get. And especially if you write your songs keeping your singing abilities in mind. Think of Dylan, Willie Nelson and to a lesser extent Cher (all great singers in their own right). You wouldn't see any of them winning American Idle though. If you like it, do it. In just about every aspect of my life I've had people tell me I wasn't good enough for one reason or another. Singing and music is one of the last things I haven't conquered yet in that aspect, but I'm having fun trying.