In spite of its bizarre shapes, heaven is still a happy place.
Parodied: Heaven is literally in the clouds; as a result, jet engines are a very real threat.
Zig Zagged: Every time Alice goes to heaven, it comes out looking different, sometimes with an explanation, other times without.
Averted: Heaven, if shown, simply isn't built in cloudbanks.
Enforced: After trying to figure out how to best envision heaven, the work starts to draw near a deadline and the creators decide to just go with fluffy clouds and be done with it.
Lampshaded: "Fluffy clouds, pearly gates... let me guess, I got the thumbs up from the big guy?
Invoked: Alice was expecting and hoping for fluffy clouds, and heaven is happy to oblige.
Exploited: As the Legions of Hell find out to their pleasure, clouds, gold and pearls are not very good wall-building material.
Defied: When a decent but rough Anti-Hero finds out from an angel he's going to heaven, his only request is "no cutesy clouds." The angel promptly sends him to the ultimate Good Guy Bar/Warrior Heaven.
Discussed: "So this is Heaven, huh? I ddn't actually think it would have the sterotypical clouds and pearly gates."
Conversed: "I can't believe the writers were so unoriginal they just used Fluffy Cloud Heaven instead of something unique."
Deconstructed: Heaven is so abjectly boring, with nothing to do but chat with angels (who are either self-righteous or profoundly alien), listen to ethereal music (that rapidly grows dull), and reminisce about past lives (which only makes the souls more nostalgic), that Hell becomes more popular.
The fluffy cloud heaven is a sort of "waiting room" for those unprepared to take the next step into the mysterious "true" heaven, which is implied to be beyond human comprehension.
The fluffy clouds might be part of the deal, but humans have carved out a niche for themselves in heaven, complete with all the amenities and pleasures of Earth (even sexual ones), but none of the cruelty or want.
Heaven takes the form of anything the citizens want it to be. Due to Pop Culture Osmosis, it often takes the form of fluffy clouds because that is what people often think about.
Implied: Bob dies, goes to Heaven off-screen, and comes back. Alice asks him what it was like and he replies, "It's basically what you'd expect. The gates were nice."
Played For Laughs: Bob dies and goes to Heaven and once there, he finds fluffy clouds and cherubs. Seeing an opportunity, he decides to have a "cloudball" fight with the little kids with wings.
Played For Drama: Bob has a hard life filled with abuse and tormoil with his only happy memory being of a cartoon he saw as a child which showed a cloud-filled Heaven complete with singing angels. He dies a tragic death and meets an angel who reveals that Heaven will take whatever form he wishes it to take for him. He decides to have the cartoon cloud Heaven and earns a happy ending.