Here are several different answers necessary, as I think we do have several different issues.
Being too fat and having difficulty to groom itself isnt not unusual, and of course, something must be done.
You did mentioned a diet. Which do you use?
Fat cats shall not starve, the liver may get hurt. (starving not so dangerours for normal weight cats, but thus dangerous for fat cats).
Dry food with low fat isnt always the best, especially if there are lots of carbohydrates instead...
Eating wet food almost exclusively is what I recommend. Good overall nourishment, and in practice being a mild banting diet.
Try to get her move more is also good... Playing.
Yes, shave or clip down behind may be a good idea. If you cant do it yourself, why, use a groomer if you want.
The real problem here is, what washes? why? Taking as example our residents. They very seldom do soil themselves on the behind. As yet, we had never washed our short-haired residents. True, they are wellbehaving boys and do wigorously wash themselves if need be, but they never get messy on they behind so we must think about bath...
So I suspect your girl does quite often have diarrhea? So this is the problem.
Normal cat poo is rather hard sausages or pears. You can held them with fingers without difficulty, hardly soiling yourself.
Sometimes they can get somewhat rare yes...
But soiling soililing, ie diarrhea - should NOT be common.
At best, try to change the food (preferably slowly - many cats dont manage to quickly change their diet nor even mark of the food).
If this dont helps - please do consult a good vet.
You will perhaps even to consider consulting a good vet already now - if you have such a nearby.
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