Basically, if you're 30 or older then you get this show and what it's about, while if you're 18-29 years old, then you take this superhero stuff (and probably many other unimportant things) WAY too seriously, and therefore can't stand how this standout episode of a very fun and funny show (I'm 45) seems to mock the "serious" shows and their fans. I've looked at the ratings of many episodes, and this holds very consistently. And although they presumably don't vote much, I bet that young children--allegedly the core audience--also generally like/love this show.
As for the mockery or satire, it's not even that, really--this episode is merely acknowledging that many of the haters of the show want a more serious one, and then the characters of course take it to a ridiculous extreme both for comedy and to reaffirm that this is what the show is about, take it or leave it. It's the typical lamp-shading the show does to wink at the audience--neither the creators nor the fans are dumb, even if they make/watch a show in which so many astoundingly (and deliberately) dumb things happen just for fun. It's not like that DC movie with the two guys with moms named Martha--an unfathomably stupid movie made for adults that took itself utterly seriously and therefore reflects the inherent stupidity of those who made it.