No, it is not appropriate to say "good morning" after 12:00 o'clock (noon). The greeting "good morning" is typically used to greet someone during the morning hours, usually until around 11:59 AM. After noon, it is more appropriate to switch to a different greeting, such as "good afternoon" or simply "hello."
Using "good morning" in the afternoon might sound a bit odd or confusing to the person you are greeting. It's always best to use greetings that match the time of day to avoid any potential misunderstandings.