Raw crab meat should have a pure white flesh with a bright red rind where the flesh meets the shell. The colors should be bright and clean. A brown color to either the white flesh or the red rind tells you that the crab meat has spent too much time separated from the shell and exposed to air.
From my experience there are two things that cause crab meat to become mushy. Either the crabs were over cooked or the crabs died and the bacteria began breaking the meat down prior to steaming. I had some crabs that I steamed and ate 1 right out of the steamer.