In theory, putting diapers under your plants should be a way to hold water near the roots and release it as needed, but diapers aren't designed to release moisture.
Keeping a dry diaper under your plants can wick away what little moisture is present in the soil, making watering less effective and leaving your plants to suffer.
Additionally, during heavy rainfall, excess water won't be able to seep past the diaper and will collect around your plants, leading to root rot and possible soil erosion.
Irrigating your garden with diapers is counterproductive and concerning because diapers can also contain synthetic chemicals and materials that may leach into your produce.
The inner and outermost layers of a diaper are commonly made with the plastic polymers polypropylene and polyethylene, which can leak microplastics into the soil and your produce.
If your garden deals with frequent soil erosion or curious wildlife, these diapers could end up all over the neighborhood, irritating the neighbors and damaging the environment.