Asklemmy

You're more right than you realize, and not so archaic really. The texture of clouds, or even clouds themselves, mostly smaller, frizzy kind of clouds are called "pilvenhattara" where pilven is a possesive form of 'pilvi' - 'cloud' and hattara is kind of an abstract descriptive word, at least today. The translation of 'rag, tatter' is a bit more complex and at least a little unrelated. There might be some historic connection, since 'hattara' is kind of a descriptive word that describes (at least for quite a long time) a kind of specific type of clouds appearance, more so a small cloud that kind of just falls apart. It's more like a frayed rag and the 'hattara' specifically pertains to the raggedness/frayed part - like the actual physical/visual quality of it being kind of frailed or jagged, like a cloud and so it does relate to clouds.

Hattara as a mythological thing is a different thing itself and again, might have some historic connection - my best guess would be that the kind of creature it means is something that is kind of 'frayed' like a vision or a fog ora cloud or something and is only seen for a moment. I'm unfamiliar with that one, though I've read a ton about folk beliefs and mythlogy here.