The line originally appeared on a South Park episode in which a foolish character kept shooting down another's ideas, dismissing them as ones the Simpsons originated, a point he made by annoyingly shouting "Simpsons did it!" The character was straightened out by a wiser character who explained that the Simpsons itself borrowed from other shows--the implicit premise being that everyone steals so no one should complain about it. The unfortunate result is that many people who look to South Park to know what their opinions are, now think that it's foolish to call people on plagiarism.
Now the line can be used more generally to ridicule anyone who points out that an episode of one show is similar to an episode of an older show.
South Park fan: Yeah, whatever. 'Simpsons did it!!! Simpsons did it!!!' That's what you sound like. Ha ha!