I had a Netherland dwarf that had this problem. She had trouble controlling her bladder, and as a result, she would pee on herself. This caused massive problems with her skin around that area. I had to bathe her heiney regularly and I clipped the fur in that area very short, but in the end I had to have her put down because she just couldn't control her urination and her quality of life was not good because of her skin problems that the contact with the urine caused.
I've also had a couple of mini rex who would occasionally get dirty rear ends, usually as a result of softer droppings from being stressed or eating something that upset their systems. I eventually cut out all "treats" from my rabbits' diets except for oats and the very occasional piece of carrot, and just fed pellets and hay, because that was the type of diet that was least likely to cause the poop issues.
I've noticed that the older does in particular sometimes start to get messy (almost all my dirty butt issues have been with does and not bucks), and I'd look into changing the diet to something more simple (this may mean switching to just hay and pellets and maybe some oats, with no veggies); looking at the type of flooring your rabbit is on and switching it to something cleaner (for example, if the rabbit is on a solid floor with shavings, maybe switch to a wire floor with a resting board so that she can get off the wire if she wants), keep the fur around her vent clipped shorter than normal to try to avoid some of the build-up, and give her more exercise. The exercise is important because sometimes overweight rabbits will have more trouble reaching their privates to clean them, and even if she's not overweight, exercising will help build up muscle strength and this can help her control her urination better, if that's part of the issue. Good luck! I know it's no fun for owner or rabbit to deal with a dirty bottom.
