I'm going to laugh at my wife for being afraid of the dark.
Why would you laugh at your wife?
It's funny that she's so afraid of the dark.
Rather than laugh at your wife's phobia of the dark, you should instead be more supportive and understanding. It's okay to gently poke fun at your wife for being afraid of a minor thing like a dark room but laughing at her can be emotionally hurtful.
Don't worry, I'm not going to just burst out laughing at her. I'm doing it like you said. Just poking a little fun at her.
Okay good, you had me worried there. Just make sure you're doing it in a supportive way and offer to help her out with her phobia if she seems like she wants to get over it.