Whatever is fickle, (line 8)
"Fickle" is not used in a negative sense, as you might use it to describe someone who is your friend one day and not the next. "Fickle" just means "changeable" or "transient," and it is true of everything in nature. According to "Pied Beauty," God is the only thing that remains unchanged.
He fathers-forth whose beauty is past change: (line 10)
Hopkins compares the beauty of God, which he considers simple, perfect, and unchangeable, to the complex and shifting beauty of the world. The question of the nature or character of God is a complicated one in Christian thought. According to Aristotle, who inspired a lot of Catholic thought, God is the "unmoved mover" who brings change to the world while remaining always and forever the same.