Agape and ajar are not interchangeable. These words individually refer to specific types of nouns.
Agape
Agape refers to the hanging open position of a mouth, specifically in response to emotion, such as wonder, surprise, or astonishment. A mouth that is "agape" is usually wide open.
Ajar
Ajar refers to the slightly open or unclosed position of an opening, typically a door. When a door is left with a small opening or "cracked," it's left ajar.