Funny-not funny
When an idea first strikes you at funny, then you quickly realize its seriousness, and the funniness disappears, leaving you only with the feeling of how not funny it really is.
Example:
Hey, wouldn't it be funny if Donald Trump actually won the election and became President of the United States?! Wait, funny-not funny.
Hey, wouldn't it be funny if Donald Trump actually won the election and became President of the United States?! Wait, funny-not funny.