'Tone deaf' is misused mostly. It's a medical condition which is easily spotted by listening to the speech of the person afflicted. We frame questions for example by a rise in pitch at the end of the sentence. Tone deaf people can't do this. So, if the singer speaks normally but can't 'hold a tune' the reason is otherwise. In such cases you can be taught to sing in tune since it's a learned not a physical impediment. Very few people who can't sing a tune are also 'tone deaf'.