My sister's cat Minnow has this exact problem. She was quite overweight at one point, but has lost a great deal of weight in the last year or so. Unfortunatly she still has a lot of loose skin which makes it difficult for her to clean her rear end.
The problem is compounded by the fact that she has mega colon and is on laxitives, which can create quite a mess! But while I would never say that she enjoys a bath, she has come to tolerate them.