It doesn't mean that they have a bad voice but yes, it does mean you're a bad singer. There is a difference between saying "I enjoy your singing" and "I enjoy your voice".

I love how there's a picture of Christina Aguilera for the 'yes' option lol.

Bad technique leads to strain and a weakened voice, and that's awful to listen to.

If by this you mean things like singing with your throat when you should be singing with your diaphragm, then yes. Using inappropriate techniques can be overcome though. That's the sort of thing that can be fixed in singing lessons.

It doesn't make you a bad singer in the way that being tone deaf would (which is genetic), and like KataraLover said it doesn't mean you have an unpleasant voice either. Just that it can be used more suitably!