This song seems to me to be a wish for being something other than you are, and a desire to be with someone you can’t be with. Why would you want to be someone’s pillow if you did not want to be by their side? Why a door, signpost or clock, if not to have a purpose, be needed? And so, it sounds as though Nick Drake thought of someone specific in this song. And the other examples (rock, pillar, statue) point at a need for strength or stability.
It only makes sense to me if the protagonist has met someone he cares about, and that something is not working out as it should. And it’s serious, it seems.

And yet, he makes it sound as light as a feather. You may just smile at the long list of could-have-beens, if so inclined.

It’s worth noting that Nick really had a way with words, in the same way that he had with music, actually. The juxtaposition of words such as kettle, door, signpost, rock are notable for their banality and functionality, and yet serve a clear purpose in the song, giving it a sense of lifelessness.