Yes, they can be if you choose the correct sticker.
Most stickers made from plastic, such as polyethylene, polypropylene, or polyvinyl chloride, are not biodegradable.
You can get paper stickers that are biodegradable & made from sugarcane fibers. See more about them here.
You can also get stickers made from wood pulp that; looks, feels & has plastic properties. These are biodegradable. Watch a video of them composting below. You can read more about these stickers here.