She means that conditions inside are as bad as the rain outside (because she has to clean up her room). " is a set expression meaning that something may be beneficial, and it will not hurt to try it, or ironically, as here, that things would not get any worse if that thing happened.
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She means that conditions inside are as bad as the rain outside (because she has to clean up her room).
"might as well ..." is a set expression meaning that something may be beneficial, and it will not hurt to try it, or ironically, as here, that things would not get any worse if that thing happened.