Is Autumn being referred to by the female pronoun 她? Is this normal?
Yes, It is a very common way to refer something to pronoun 她 in Chinese, commonly known as personification. In such a way, the author can strengthen his/her feelings about autumn. There are also many similar cases: such as 长江/黄河/故乡(Yangzi River/Yellow River/hometown) can often be referred to 她 as well.