Tetris is actually a direct sequel to Monoris. You don't need to have played Diris or Tris to understand it (which are standalone) but you miss out on some of the lore. I definitely recommend playing the whole series in order if you have the time, though. People will tell you to skip Diris but I actually think it's the best in the series.
For me it's Diris > Tetris / Monoris > Tris. I will never replay Tris. It's fun to speedrun but that's literally it. I don't understand how people can hate on Diris but think Tris is somehow better??