She has what is likely a food allergy. The vet says that as long as she seems to ignore it, we can too. It's basically something like kitty acne, annoying and unsightly, but not life-threatening. If we want to confirm for sure, we can try a prescription food, but they were out of it, so I'm going to talk to mom about switching to another brand, maybe one of the higher quality store brands, before we use a vet food.
I checked, and we did recently get new food for the cats, so that makes sense. Since we feed the outside cats too, mom tends to go for the stuff that comes in bulk, and I'm thinking that something higher quality might be a good idea for our indoor cats who can't opt to go catch a mouse.