Garlic is not good for cats.
There are two potential problems with the use of garlic as a flea or parasite preventative in pets. First and most important is that garlic is potentially toxic. Second, in my experience when used to prevent fleas and parasites garlic is completely ineffective.
Garlic and onions contain chemicals that are oxidizers. These chemicals can react with the blood of pets (especially cats) to cause a syndrome called Heinz body anemia. Onion is much more dangerous than garlic. However, chronic toxicity studies of garlic have revealed cases of anemia (source: Handbook of Small Animal Toxicology and Poisonings, Gfeller and Messonnier, 1st ed.). Garlic also can cause skin rashes and breathing problems.
Garlic-based flea preventatives are commercially available despite their inefficacy and the risks they pose to pets. I suspect that this is due to lax government regulation of pet products.
Just because something is natural does not mean it is safe. There are millions of natural carcinogens, endocrine disruptors, mutagens, and toxins. Cobra venom, aflatoxin, and botulism toxin are all completely natural–and completely deadly.
Garlic certainly isn’t in the same league as the poisons listed above. Some pets appear to be able to tolerate garlic quite well. Not every pet suffers anemia or other problems after consuming garlic. Nonetheless, I cannot sign off on the use of garlic in pets.
Chronic diarrhea in older cats has many causes. Thyroid disease, inflammatory bowel disease, and parasites are just a few. I’d recommend a trip to the vet for your cat.