First there are some grammatical mistakes. The expression is 'might as well have been'.
So your sentence should read 'She might as well have been a skinny doll, made of reed'.
Your second sentence means almost the same thing, but needs 'a' (indefinite article) before 'skinny'.
The two do not mean exactly the same thing. The first is not necessarily saying that she 'looked like a skinny doll made of reed', just that she might as well have been one. This is important because the term is often used metaphorically.
I might say, in a moment of exasperation, cleaning up after my kids ' I might as well be a skivvy'. It doesn't mean that my life is necessarily like that of a skivvy. It is just means that it feels like it at that moment.
It is used in many other senses e.g. if I take an umbrella and it doesn't rain, I will say 'I might as well not have brought it'.