Just speaking from my own experience, but dry messy bottoms are the ones that I'm referring to. Wet messy bottoms are the ones that tend to indicate something is going on in my experience. Sick birds tend to have wet messy bottoms as opposed to dry, but that doesn't mean all birds with that kind of messy bottom are sick. For instance, sometimes some of my free-rangers find and eat something that gives them loose droppings and their bottoms end up messy. Or, if it's very hot and the birds are drinking a lot, the same can happen. In neither case are the birds actually sick, just something happened to trigger it. Wet messy bottoms are the ones that draw in flies and can quickly lead to fly strike, so those need to be cleaned up pretty immediately. Dry messy bottoms maybe can wait if you're busy, but should be cleaned up at some point as they can end up turning into these big masses of poo that can rip feathers out or, in extreme cases, block up the bird so that they can't pass any more droppings or even eggs.