You can leave your mobility scooter outside as long as you don't live in a town or city with extreme weather. If you live in cold weather, it's best to keep your scooter under a roof, preferably in a temperature-controlled space.
The battery may suffer if the scooter is constantly exposed to cold weather. If it must be stored outside in cold, you may choose a travel scooter. These scooters have lightweight removable batteries that can be stored inside while the scooter stays outside.
No matter what the exterior temperature may be, it is still important to keep the scooter dry. A shed or a large tarp may be enough to keep your scooter safe and functional if stored outdoors.