The episode received generally positive reviews. Travis Pickett of IGN gave it an 8.5 rating, especially praising Trey Parker and Matt Stone for managing to contrast the episode with the actual Simpsons with themes like Cartman performing fellatio on "some guy in an alley", while respectfully paying their dues."Simpsons Already Did It" was inspired by the fact that The Simpsons did in fact beat South Park to several plot concepts. In the season 4 episode "The Wacky Molestation Adventure", Butters was supposed to block out the sun, but one writer pointed out that "The Simpsons already did it." The episode "calls out" the obvious observation that The Simpsons have realized a vast number of ideas throughout their long-lived run. Some have found a certain reciprocity to this statement, finding instances of repetitiveness in The Simpsons itself while quoting South Park. However, although the motif throughout the episode is "Simpsons did it first", South Park creators released their feature film based on the series in 1999, eight years prior to The Simpsons Movie. The Simpsons crew has a friendly relationship with South Park, which they demonstrated several times, going as far as sending flowers to the South Park studios when South Park parodied Family Guy in the season 10 episodes "Cartoon Wars Part I" and "Part II". In 2010, The Simpsons crew congratulated South Park for reaching 200 episodes, with a message reading "Congratulations on 200 Episodes. (We Already Did It.) (Twice.)".
Soon after, in reference to the controversies and terrorist threats surrounding depictions of the Muslim prophet Muhammad in the South Park episodes "200" and "201", the chalkboard gag on that week's The Simpsons episode, "The Squirt and the Whale", read "South Park – We'd stand beside you if we weren't so scared".