Interesting question. I'm so curious exactly what you did and with what benediction in mind. I think there are many very interesting customs and practices. Some have underlying practical effect, some are purely spiritual.
Realistically and factually though, I suppose if the apple core was simply tossed onto the surface, it might be discovered by some wildlife. In my garden, I imagine squirrels or chipmunks will get first dibs, though in somewhat wilder garden, deer, groundHOG, and maybe skunk and turtles or even fruit rats may be other possible receivers. Some fruit loving bird may find the opportunity.
If not animals, insects like ants, bees, wasps, flies and butterflies may find the offering while fresh and fermenting/rotting, while soil detrivores like slugs, pillbugs, sowbugs, earwigs, and earthworms/nightcrawlers may find it delectable as well.
I really don't think anything of the apple core will be left to harm the plants in your garden directly unless the core landed on top of a seed or seedling and damaged or smothered it. If any harm, I suppose the attracted animals or insects may potentially cause damage, but just as likely could assist as a predator and maybe a little fertilizer.
If any of the apple seeds survive and are buried, they may sprout next spring. Whether you would consider this harmful -- a.k.a. weed -- or beneficial is up to you.