By definition things seem more boring than they are. Rosaluma doesn't look like a Witch. So she isn't one in my book.
Is Lady Palutena a Witch too and not a Goddess?
A witch isn't defined by aesthetic though (honestly by that logic Bayonetta looks much less like a witch out of all of the witches I've seen, at least Rosalina has a wand, which is stereotypical of witches).
And it's funny you make that comment about Palutena because in the Japanese version of Rosalina's Palutena's Guidance, after Palutena calls Rosalina a witch, Viridi says "Hey, if she's a witch, doesn't that make YOU a witch too?" (though please note these are rough translations from someone inexperienced. I can assure you the word "witch" keeps popping up in the conversation though, and that my statements are true, if not the exact translation. I have heard other people say Viridi also mentions Zelda as well as Palutena in the comparison, though I haven't noticed that myself).
There is no reason why one can't be a witch and god at the same time, since the former is defined by whether or not they can use supernatural powers, and the latter just speaks for the EXTENT of their powers and whether or not they rule and are worshipped as an authourity figure.
While I tend to just call Rosalina a "goddess or something more" myself on forums and such, if I was writing an article on her character or something, I would call her "a witch with god-like ability, experience, potential, and responsibilities". While she is the creator, watcher, and protector of multiple universes, and is powerful beyond belief as a result, she actually doesn't ever present herself as a moral authority at all, really only being concerned with whether or not there is a universe to live in and not defining certain things as moral/immoral and enforcing this with an iron fist. It seems she only interferes with the affairs of mortals when the fate of the entire universe is at stake. Her official bio on the amiibo website seems to back this up, by stating that "she guards the cosmos against any and all threats".