If it were できた, it would sound as if the ripple pattern in question formed at the moment the narrator looked at the sky, rather having already been there.
It's the same as 家に着くと、窓が開いていた versus 開いた. The former describes a situation where the window is already open when the speaker gets there. The latter would mean the window had been shut before and suddenly opened in that moment.
It could be できている if you wanted to shift the tense to the present, e.g. to create that sense of immediacy or what-have-you as has been mentioned throughout this discussion.