Here comes the music historian on "How Nightcore is NOT a music genre".
Mid 90's, everyone was going nuts over anime. Pokemon was the fucking tits, and everyone loved the games and the show and the movies.
Two norwegian students set up a school project, they formed a group called Nightcore and with limited music trackers and stolen samples from cartoon VHS they made a Happy Hardcore song alongside a video, quickly it rose to fame in Newgrounds just like Animutations (mainly because anything Pokemon or KOF would get views) and became viral. Eventually people would do colorful videos and use happy hardcore songs (which are VERRRYYY hard to get so they used Speedycake remixes) and so, anything that was high-pitched and more frenetic than your usual snobby-house track or yuppie trance track was called J-Pop or asociated with japanese and anime/manga culture and eventually called nightcore. While Happy Hardcore and J-Core crossed borders with DDR and beatmania IIDX (like Osu! but harder), Japanese electronic music set itself as it with groups like capsule, Perfume, anything shibuya and whatever j-pop trend there is, Hello! Project or the likes, the term Nightcore remix rose like bad weed with sped up versions of it, mainly to emulate Happy Hardcore and related.
Itself is not a genre, because all of their song aren't actually composed in any way, lacks any characteristic perk, technique or sound and stuff. While other mock genres like vaporwave evolved from slowed down tracks to more complex sounding pieces, usually bordering the likes of musique concrete or even chillwave and downtempo, nightcore does not feature any intervention, sampling, processing aside the "paul stretch" or anything whatsoever.
tl;dr you're not a musician or dj for speeding up songs in audacity