Maybe I am also not a vocaloid historian but like Rolling Girl by Wowaka and other where already there like 13 years ago, but I would not use the world cringy to describe the sadder side of vocaloid, yeah there is some but a lot of them are very good too.
Hastune Miku released in 2007, dunno about how popular it was on release but I see quite a lot of depressing/sad stuff from 13 to 10 years ago, so it might not have been day one but pretty fast it was part of the identity of the scene, then again idk when the Vocaloid scene actual did start
Edit : to add to this, Vocaloid did give a voice to the voiceless, a lot of people may want to express themself to sing about what they felt and when through but did not bring themself to do it so when the Vocaloid came up that opened a flood gate, I could go longer about that but I am bad at word