A physically ugly/unattractive person likened to the sometimes messy aftermath of cracking open the shell of a freshly cooked "muddie" (mud crab for the Un-Australianised).
Bob: So what's she like?
John: Great personality, but she's got a head like a bashed crab.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.