"Am I supposed to get up on something for my Phoon picture
to be accepted?" No, there is no such requirement.
"Is it more interesting if I'm up on something?" Hmm. I suppose that depends on what the thing is.
It was funny to me when good friends John and Gretchen told me that seeing the slab of concrete on which they're standing is what motivated them to Phoon in front of this beautiful cathedral. That illustrated the common misconception that phooning is somehow tied to being up on something.
With that said, there are things that seem to call out, "Phoon on me!" The better examples of "up on something" photos are the Had to Phoon on it ones.
It is often true that climbing up onto something in the view would be more interesting to everyone's eye than just Phooning next to it. And there are always those who will take this idea to the extreme.