That's a practical question! For a standard outdoor trash bin, the lid isn't designed to hold a person's weight. Most residential plastic or metal lids are meant only to cover the bin and keep pests out. Their weight capacity is typically just a few pounds – enough to withstand wind or a curious animal, but not an adult or child sitting down.
Attempting to sit on it poses several risks. Plastic lids can crack or shatter suddenly, leading to potential injury from sharp edges or a nasty fall. Even sturdier metal lids can bend, detach, or cause the entire bin to tip over. The hinges and attachment points are the weakest links and are not engineered for that kind of load.
If you need a seating surface outdoors, it's always best to use furniture specifically designed for that purpose, like a bench or a sturdy stool. Your standard trash bin lid, while handy for its intended job, just isn't built to be a perch. It's safer for you and the bin to keep the lid for closing the garbage.