Flying all over the sky is usually used to describe the scene where lighter substances such as snow and dust fly in the sky, giving people a light and elegant feeling.
Flying all over the sky is often used to describe the scene of light substances such as snowflakes, catkins and light petals flying in the air, giving people a beautiful and elegant feeling.
In a word, both of them can describe snowflakes or light objects flying all over the sky, but they are slightly different in specific usage and context.