No. Diarrhea is liquid stool that should not go on for more than 3 days. Soft stool is not harmful and usually means that your cat is overeating and not completely digesting everything they are consuming. Soft stool could be due to a variety of medical conditions, perhaps a parasite.
When a cat overeats, the improperly digested food dilutes the fiber that makes the stool firm, causing it to soften. Same thing happens to us when we eat too many cheese curds at the state fair. See our article on transitioning your cat to a new food.