Peanut butter is a food that both humans and cats can eat. However, it should be consumed in moderation by your cat. Peanut butter is not necessarily bad for your cat, but too much of the food could cause an upset stomach or other health problems.
Also eating peanut butter doesn't provides any nutritional value to cats and If ingested too much at once, it will result in vomiting or diarrhea or both.
Use peanut butter as a treat for your cat and give it in very low quantity.