I reckon so. I'm learning the guitar and I'm not too bad, yet my singing is abysmal. I think you can appreciate when a noise outside of your body (i.e a guitar string being plaucked) is off key, more easily than when you make a vocal sound and it's all internal (i.e vibrations through bone etc).
Yes, most guitarists read their music from tablature rather than a musical stave, so you don't need to be able to read music at all, unless you're playing classical guitar. I can't sing at all, but play the guitar pretty well because, like mortartube says, it's easier to hear if something is out of tune when it's not coming from yourself. I'm not sure if the same applies to most instruments though, as I think you generally need to be able to read music to play them.