I've been using my phone to record my speedruns, including Xbox One, and needed to know if using Mixer is easier for the Mods than Youtube or Twitch

Not sure if this website has auto-embed enabled for Mixer highlights, but you should definitely just do what you want to do, as logn as it's valid, free, and what suits you best.

But, as a leaderboard moderator, I'd be happier with YouTube :)

Local recording is always a good idea.