Guppies, platies, and all livebearers in general, do NOT eat plants. They do eat microfauna and bits of algae that grows on the plants.
They are gluttons and are constantly grazing. That's their role as omnivores. They are constantly on the look out for food, and live plants provide a wonderful habitat for microfauna and algae to grow, which they will nibble on constantly. If you are noticing dead bits of plant where the fish had been snacking, its likely due to algae growth that blocked light in that area of the plant and the fish are eating the algae that has grown there.