Have you had your kitty checked out by a vet? There may be some underlying medical problem as to why she is overweight. That is a good place to start. If you are already giving her diet cat food, make sure it is the proper amount for her. Make sure she isn't getting table scraps from you or othr family members. I have a "fat" cat...well, he USED TO be fat, but has some medical problems now (kidneys) and has lost weight., but when he was fat, I had to have a special "butt rag" that I would use on him because he couldn't clean himself either. I had to use it daily, or a couple of times daily, wetting the rag and wiping him off. I gave him baths too, and to this day, he is a trooper when it comes to baths!!! Good luck, but if you get the obesity under control, I am sure your kitty will start to clean herself properly....kitties don;t like to be dirty.
Have you had kitty checked for weight related issues? The best way to deal with this is to help kitty lose weight. Exercise is the best way, along with a strict diet. What diet food are you using?
The trick is to get kitty to run around for a half hour once or twice day. Now, depending how overweight your cat is, this won't be easy.
You're going to have to be inventive and come up with something to get your cat to move around. If kitty won't play typical string games, you may want to consider a leash/harness and taking kitty outside in the yard or park. Don't try to walk him, but do try to get kitty to move around. A cat so overweight it can't properly clean itself is at serious risk for various health issues.