I think we need to be careful to answer the Question properly here - are you suggesting people with genuinely bad voices, or artists who voices you don't like, which is not the same thing.
For example, I loathe Whitney Houston's voice with a passion, but only because it is all technique and volume, over emotion and feeling.
So I would say that I don't like Whitney Houston, but I cannot fault her vocal technique and skill because they are peerless.
On the opposite side of the coin, you could never say that Leonard Cohen has 'the gift of a golden voice' as he ironically admits, but I absolutely adore listening to him because of the way he sings, and what he sings about.
So, make sure your choice is a bad singer, not a good singer you don't like.
Of course, you can combine the two with Chris Martin, who sounds like a nervous fourth-former at a school concert, and contrives to write utter drivel to sing as well.