Personally, my working theory is that the reason people decry the Western fanbase's music as edgy stems from the fact that it's different from what they expect.
I'm not Japanese and I only know bits and pieces about Japanese culture, but my impression is that there's a push there toward people getting along, being part of the group, and dealing with painful/difficult stuff in a way that's quiet or invisible. (I've put together this impression from things like the admonishments you see regularly enough in anime for people to try to smile and make the best of things, as well as from stuff I've read about some schools there having rules about uniform hair color, and possibly other things.) I imagine this is part of the reason why happier songs are easier to come by from Japanese producers, as well possibly as why songs about difficult subjects are sometimes sort of masked inside upbeat melodies.
(Edit: I mean, isn't that basically what "This Is the Happiness and Peace of Mind Committee" is about? The lyrics don't really tell us if the song refers to a cultural mandate to be happy, or if it's some other mandate to be happy. But it's obvious that happiness is being forced. And all of that aside, it's certainly a song with dark lyrics inside an upbeat melody.)
In the west, by contrast (or at least in the U.S., if I'm honest and talk about what I know better), we don't have that kind of push to that degree. It seems like there's a wider range here within which people can be different and still not be regarded as outright weird, dark themes in conversation/media are by no means hard to come by, and complaining about what-have-you is almost a conversational style (Actually, it's a decent way of getting by in awkward social situations, if you can find something that both you and the strangers around you commonly dislike. Is the weather poor? Is traffic bad? Is the local sports team performing terribly? Congrats--you have instant camaraderie.). In any case, westerners seem more open about these less-palatable topics, and I think that gets reflected in the music they make. At least that's my theory.
Continuing from there, the Japanese fandom is where the roots of Vocaloid/vocal synths are, and lots of its iconic/famous songs, as well as most of its songs by percentage, come from there. So when a new fan goes on YouTube, for instance, he/she starts listening to stuff and most likely hits a stream of happy Japanese stuff. And then, just as they're thinking that this is fun and good, they run into something from the western sphere, and it quite possibly winds up being a lot different. Looking into a few other western songs, they soon enough find that there seem to be a lot of songs that run this way, and they develop a negative view of the western fandom in general.
Personally, I don't think that edgier songs necessarily are bad things. Positive songs can lift people's spirits, or possibly provide an example or ideal toward which people might be inspired to work. At the same time, edgy songs can help both the artist and the listener cope with painful things, or the songs could contribute to beneficial changes in the broader culture by pointing out things that aren't working. So they both have the potential to do good things, even if they come at those things from different angles. (In a way, it's sort of like the difference between Superman and Batman, where the former is this straightlaced, lead-by-example type, and the latter works under the premise of "scare the crap outta the bad guys so they don't do bad things again.") Vocal synth music is just a part of the larger umbrella of music with vocals in general, and music incorporating vocals already spans a large spectrum of happier or darker stuff. As we do with music sung by humans, we should just pick the music we like.
They've always been around, it was never unique to Western producers, and this discussion has cropped up multiple times in the past on this thread.
