Not sure if you meant it as a joke, but I had this same question. Is this sentence ambiguous and you need to know by context whether you're referring to the weather or a human? And not even necessarily in a dirty way, a human can be soaked.
For that matter, I am curious if fliuch is used in an adult context as well if that doesn't translate.
We don't have a specific word for "it" in Gaelic, so we use "e" and "i", the words for "he" and "she". Which one you use depends on the grammatical gender of the noun it replaces. There's more info in the lesson notes, which can be seen on the web version of Duolingo, or here:https://duome.eu/tips/en/gd
It can be - weather is considered feminine, therefore when using weather phrases "i" is used to render "it". So in theory this sentence can also mean "she is wet" - but since it's a unit on weather, it's "it" that's wanted, in general. Though "she is wet" ought also to be acceptable. If you get marked wrong, report it as "should be accepted".