Ok, according to Digital Spy, Jonathan Blow confirmed it's rather cosmetic meaning:
One of the game's mysteries was a stationary cloud positioned beside the castle in the epilogue, which Blow explained has no impact on
"There are Braid conspiracy theorists out on the internet who swear
that this cloud is for something, and the reason that they think that
- and there's a very good reason to think that - is that pretty much everything else in the game has a reason," [Jonathan] explained.
"The reason why this cloud is here is not very verbalisable to me.
It's something about victory, it's something about having a beautiful
vantage point that I can just stand on, and feel successful about. It
just felt right that my castle not only encompasses some blocks that
were made, but also some elements that were previously antagonistic to
"I just wanted to be in a position where I could look down on the
text. I wanted to be high up looking down. Just when I build a castle
like this, or added a rampart, it just didn't feel the same. So again,
it's a pattern break, it's like everything in the game means something
except for this."
He added: "I guess I've never answered that officially, but there is
no gameplay purpose to this cloud."