There are some hens that tend to get feces accumulation on their vent feathers more than others simply because they have more fluff or possibly digestive issues, but yes, feces accumulation is common with sick birds since they tend to produce watery stools, and are usually inactive the feces builds up faster. If she is laying eggs, acting normal, she probably isn't sick, but it might be good just to do a quick physical exam, check her crop, mouth, check for ectoparasites, etc.
Could you post a few pictures of her vent? Excessive accumulation of feces around the vent area provides a perfect environment for flies to lay their eggs so it would be a good idea to grab her, trim a little of the fluff back. Note, if you trim the feathers down to bare skin her feces will then start falling onto her bare skin, which may cause vent gleet, so just trim the long feathers, and do a good wash afterward.