This is the usage I have seen primarily - putting one's foot in one's mouth or otherwise making some sort of harmless brainfart, or used to describe someone who often makes such minor blunders which are less about being "stupid" and more about being thoughtless or distracted. When it's used to actively mock, I've mostly seen it used to mock people spouting bullshit, like "men's rights herp derp derp".

BUT. It bears a lot of resemblance to other types of mockery of the developmentally disabled, and I am seeing more and more instances of it being outright used that way. Within the show, and within some of the fanworks, "Derpy Hooves" is a ditzy, possibly forgetful pony, who is otherwise treated with respect and seen doing ordinary things (as much as she is seen, since despite her Ascended Meme status, she's still technically an unnamed background pony), but there are definitely a number of fan works that to no treat her this way, and are offensive and mocking towards people with intellectual disabilities (and physical ones, as well, since the name came from the alignment of her eyes).