When Charlie was a kitten, he had some soft stools, and the vet recommended giving him a little bit of plain yogurt. As far as I remember, it did help. Most yogurts have live and active cultures, so they’re similar to a probiotic. I would recommend asking your vet to see what they think would be better. Did your kitty have an appointment about the diarrhea? If not, I would definitely advise you to make one. There can be multiple causes for cats having diarrhea, including intestinal parasites, so it’s best to get it checked out.
I think yogurt is OK, but just tiny amounts are probably all that is needed. I was mixing a single tsp. of plain yogurt into some meat baby food and everybody seemed to like it. I did that for a number of nights.
I had just finished a week long dosing of an anti-parasite med called metronidazole - which I was expecting to kill their natural flora. It has taken about a week for things to firm up around here.