Nah... it's mostly that while there are unambiguously good mythical critters in japanese folklore, they're by and large not yokai, so far as I can recall. Little kami, other sorts of spirits, stuff like that. Yokai, as noted, are almost always either malevolent, massively irritating, or have some sort of trigger (ala the kappa food bit you just mentioned) that fires those two up, with very, very few exceptions (though there are some exceptions. They're just, y'know, exceptions.). Is why I'd call the closest equivalent to them to be the older tales of the fae. Yokai has a distinctively (albeit often mildly) negative connotation, from everything I've seen. It's telling that even in more modern stuff that has yokai presented in a positive light (or at least positive role), they're still pretty likely to be... kinda' jackasses, at their best.
Personally, I just don't bother translating it in my head, if I run into a translator that doesn't process the word into english. Yokai is a particular category of critter that doesn't really track terribly well to a single word in english, so translating it just leads to mild confusion, imo. Spirit would probably be closest, but you'd probably need to add dangerous or trickster spirit to that, something along those lines.