In the United States, there are a surprising number of rules and regulations regarding what you can do with your own mailbox. For example, homeowners are not allowed to attach fliers or posters to their mailboxes.
With that said, most decorations are fine as long as they don’t interfere with the safety of the mailbox or prevent postal workers from using it.
Before decorating your own mailbox, take the time to research any restrictions in place in your own area!