As a layperson, I've only heard of alum being used in water treatment facilities, IIRC (I think I read about this in a Magic School Bus book when I was a tadpole).
The era of cartoons you've observed gives the key here. There are a number of different variations of the alum compound, but the fine white powder you see in the cartoons is potassium alum [KAl(SO4)2 12H2O] which was commonly used in pickling and was readily available in grocery stores for that purpose. As home pickling became less common, the widespread recognition of the product decreased, meaning this gag isn't used now.