I'm not at all pushing a particular type of diet but something that I've noticed with 100% raw feeding is that Atticus, my two year old grey tabby, has a spotless derriere.
Before the switch I was feeding him a rotation of premium grain free canned food in the morning such as Koha kangaroo, Weruva Paw Lickin' Chicken, etc., and although he didn't ever have anything in his fur, his actual, er, "kitty rectum" could be a bit, shall we say, crusty. Nothing monumental but for hygiene's sake I would take a kitty wet wipe to it at least once a day. Now that he's on 100% raw his stool is much firmer and smaller, and I've not noticed anymore crustiness at all.
Is t a long haired-cat? If yes, you may want to consider shaving the bum area so poop doesn't get stuck there.
Ask your vet to do it on your next visit or bring it to a groomer. If it works, you may want to consider investing in a trimmer so you can trim it yourself. Just don't shave too close to the genitals for obvious reason.
Also, if a cat's poop is too soft, that will increase its chance of getting stuck in the fur around a cat's bottom. Maybe make a quick call to the vet?