Cats are expert groomers, except when their weight doesn't allow them to do much except swing their paw in the air when they try to bathe themselves. A lack of grooming wouldn't be so bad if it didn't make for a poor coat, a smelly behind and a pimply face.
With regular grooming, your kitty maintains a good-looking coat that's fairly free of tangles. When she can't groom herself, her coat turns dry, scraggly and mats will start to pop up. Mats are clumps of hair that are twisted and tangled, sometimes right down to your kitty's skin. As you can imagine, they're painful. Anytime your feline brushes the mat against something or kicks at it with her back claws, the tangled hair pulls away from her skin, leading to irritation. Sometimes an infection can develop, and that infection is worsened because of the dirt and nastiness that gets caught in the matted hair. The problem is especially common in long-haired cats such as Persians.
Maybe not the most dangerous side effect of poor grooming but certainly the smelliest, smidgens of feces that don't get deposited into your cat's litter box can instead catch a ride on her fur. Normally, your cat would deal with this by tending to the area. But since she can't reach her butt, you're left with a perpetual smell that might have you reaching for a breathing mask. The waste can harden if not removed in a short time, and your cat may feel it on her bottom, causing her to scoot across the floor in attempt to remove it. Note that butt-scooting might mean her anal glands need expressed.
If you start to see small blackheads start to appear in numbers under your kitty's chin, don't be surprised. A lack of grooming opens the door for feline acne, and the problem isn't only an aesthetic one. In moderate to severe cases, pustules will form and your kitty might rip them open trying to stave off the insane itchiness they cause. If that happens, bacteria can move on in and create an infection.
Since your overweight feline can't groom herself, the duty falls on your shoulders. Brush her daily to keep her coat smooth and free of mats. As for the stinky behind, scoop her up and wipe her down with a lukewarm cloth -- that's the only way to get her back to smelling pleasant again. You may want to use a bit of kitty shampoo on the area. You can also take her to the groomer and have her trimmed back there, especially if she has long hair. If she gets dirty, dusty or smelly elsewhere, bathe her. If she freaks out at the sight of water, opt for a waterless shampoo. If she already has mats or later develops them, try to gently and slowly brush them out. If that doesn't work, drive her to a groomer. Do not cut them out yourself, as you might inadvertently cut her skin or pull the hair out painfully.
Unless your vet has told you her weight is causing her poor grooming habits and has set you up with a diet plan, make a vet appointment when your kitty stops grooming appropriately. Poor grooming habits can be caused by an illness. If her weight is the issue, you need to start her on a new diet to slowly trim her down. Even if she has a condition that causes obesity, such as hyperthyroidism or diabetes, you should be able to get her back to a healthy weight with a diet change and medication.