The development of the meta of the meme became adding him to more and more outrageous hiding spots, which isn't really captured on knowyourmeme.

I think this works because it's a surprisingly good hiding spot because he sort of blends in to the aesthetic of the image, but is silly for obvious reasons to anyone who's tried hiding inside a forearm.

Also the fact that he was snuck into images like this makes his appearance come with a sort of "gotcha" moment, a little like recognising Loss or similar memes. The humour is for people who've seen it before, but not for a while, who go "haha mouse... Oh goddammit" when they see it.

The fact that Saddam Hussein was explicitly labeled in a USA Today graphic is funny.