That's really interesting, BobK!
I certainly agree that weasels aren't ever going to be seen as a "nice" (such a weasel word, pun intended) animals - after all, they were the "bad guys" in The Wind in the Willows (and various other children's books) so the evil connotation is clearly still present perhaps subconsciously, and even nowadays we use it as a word with negative connotations when referring to things other than the animal.
However, I still don't think they are "scary" in a meaningful sense: the language of the novel is very modern American and the intended audience is teens from what I can gather. I would be inclined to stick to my guns and say that weasel was intended primarily for its incongruity as an oath, and the resultant slightly comic effect. After all, were I to start using "Ten thousand furies and serpents" as an oath nowadays, I suspect (far too) many people wouldn't understand the reference - in fact I doubt one in a thousand would - and find it comic rather than serious.