While Maddy sounds like an okay nickname for a little girl, Matty is horrible for a little boy. Of course the name is far too infantile for anyone past the age of, well, 8. In American pronunciation, this sounds feminine, and thus weird on a boy.
That's my brother's nickname and he's 10. It's not horrible and it makes him different to all the other Matts. He's never had any trouble with his nickname. I think it's a nice nickname. :)
It would be cute, but it does tend to sound like the feminine name "Maddie" and every time I hear it I think of something being matted down. I don't know, maybe I'm just weird...