@Fuulie: You say that as if it's a bad thing, LOL XD!
Anyway, I don't really like meme-speak too much for the reason you said and what others have said (also the reason I said, haha).
On the original topic, herp derp does mean a lot of things...the majority of which are just pointing out stupidity (especially "derp"). It also somewhat started in which...while you are watching TV, and you pause at any given moment, the person speaking is most likely going to have their mouth open in some strange fashion and probably in half-blink, giving the visual impression of a severely mentally handicapped person.
Sometimes regular pictures are photoshopped to have eyes crossed inwards or outwards or pointing in random directions to enhance this effect.
It's kind of creepy if you ask me :P