To a first degree approximation, it's the actively-developed fork of mplayer.
It is mplayer but better.
mpv is the successor to mplayer2, which was the successor of mplayer.
You can use --vo=tct if you're on console. Also I heard MPV has Sixel support now, but I couldn't get it working when I tried.
I'm sorry if this is obvious, but did you double-check your mpv was built with Sixel support? `mpv --vo=help` will list the video outputs your build of mpv supports; sixel should be listed there if your mpv was built with it, but I suspect most distros package mpv without sixel support.