I'm not super familiar with Moomin; Mika's hat looks kind of like the one Snufkin is depicted as wearing, but the rest of her outfit looks different enough that I don't know if it counts as cosplay. I wonder if the hattifatteners are supposed to be inspired by some kind of fungus.

Edit: Looking at https://www.moomin.com/en/characters/hattifatteners/, it does describe them as mushroom-like, but they move around.