I think you missed my point on narrative entirely. ;.;
As you perceive the world, each person has knowledge, biases, and morality that filters the world different from any other perspective. And Ooo, as the viewers perceive it, is Finn's perspective of Ooo.
The best analogy I can sum up is a comparison to Alice in Wonderland. Alice is experiencing this strange world, or other's say, our own world, and interpreting or imagining these things for the reader. We can't say it's a dream, 100% reality, or reality as per Alice. Or 'through the looking glass' into Alice's perception of the world.
btw, I don't think Ooo is a giant Finn dream and I believe these events are actually happening, but is it 100% reality or Finn's reality? Is that the real Marceline or how Finn perceives her? It's more conceptual than literal. Finn's not observing every moment, but his perceptions play out into the world (because writers can fuck with you like that).
I'm sure there's some filler, but they tie these episodes to plot-heavy arcs with perspective narrative. This is all about Finn's developing perspective -- you see the emotional aftermath of major plot points during these 'filler' episodes.
The best examples come from character involvement like Tree Trunks, PB, Ice King, etc. Best one is PB since there's a lot of emotion there. Have you ever noticed her character develops around impasses in Finn's life?
Start -- super chill, except quick to anger since Finn is embarrased
Breaks Finn's heart -- reclusive, more into science than people
Into Flame Princess -- non-trusting, overbearing mother, getting into other people's business
Getting Over Flame Princess -- all of a sudden getting super chill and letting go?
I mean it's not FACT, but re-watching the series brought a lot of this into perspective (heh) for me. Adventure Time is not only a cartoon show or a storyline, it is us, the audience, experiencing our coming-of-age in the land of Ooo.
Actually found another post that puts the coming-of-age and themes of Adventure Time are linked: