It is a joke, but not an inside joke as much as a joke that would be understood by people from Balkan countries (Serbia, Croatia, Bosnia). Miran (the author of LYAH) is, judging from his first and last name, from this region.
In the local language (Serbian, Croatian and Bosnian are basically the dialects of the same common language), the most commonly used word for teach and learn is the same word, uciti, although there are separate words for these activities. Basically, whether you speak of teaching someone, or learning something is judged by the context.
Then, Learn you a haskell vs. teach you haskell is a wordplay in that sense.
A similar joke from the region: "Can you translate me to the other page of the street?"
The author is a very funny/insane guy. Exhibit A is his twitter account (https://twitter.com/bonus500). I wouldn't call it an inside joke, but rather a general feature of good comedy where it feels familiar yet distant.
It is a gamer|anime|comic book joke. There is a lot of cheaply written entertainment with poorly defined motivations ascribed to the heroes and villains. So what battle cry is used by an overly generic hero? Why an overly generic cry of "For great justice!" of course. Throw in a joke a about poor translations and you get their title.
I believe the title is a riff off of "Learn You Some Erlang for Great Good!" Some other books or online tutorials have taken that form of title. I don't really know why, though I suppose the silliness offsets the otherwise heavy nature of the content. (Learning Erlang is probably easier than learning Haskell for most people, but it can still be a mind stretcher if you've never done a functional language before.)