I don't wanna be that guy, but Yorick's ghouls are pretty difficult to last hit at the rate that they degenerate HP. Add to that your cs was actually quite low in that game.
234 CS at 50 minutes is very low. I get that keeping cs numbers up later in game is difficult and that as someone who also struggles with cs numbers I don't have a lot of room to talk, but running an average of 48CS/10 minutes of the game is very low.
If I give you an example from one of my games, it was a 22 minute surrender, my cs numbers were 173 for 22 minutes, which is still about 50 short of where it should be, but that amounts to 78cs/10 minutes of the game.
If you look at your lane opponent, that Riven had 305 CS to your 234 CS at the same time. That's a 71 CS differential which amounts to about 3 kills worth of gold.