Most criticism in League is pointless. Rarely will you be able to enact some behavioral change in time to make the player perform well.
There are exceptions, of course, but only if you phrase your criticism in such a way that your teammates won't feel defensive. And if you can't do that... then maybe you ARE flaming. Because if you're criticizing for the sake of making yourself feel superior, then that's negative and pointless (I hesitate to call it literally "flaming", but it's in the same ballpark).
Example of productive criticism that I've personally said in games:
"Hey, Ori. I gotta stay in lane, I can't help you effectively if you invade. You're gonna get caught and killed if you keep doing that"
"Hey Nasus, your Q stacks damage infinitely if you last-hit with it. That's how you'll be strong late-game."
Both players were acting like idiots, but I'm not going to accomplish anything by calling them idiots. The Nasus had no idea how his kit worked, but I informed him and he at least got several hundred stacks by the end of the game. The Ori had died in a dumb spot twice, but stopped doing it after I respectfully pointed out her error.
Example of unproductive criticism that has been pointlessly said to me:
"Land a fucking hook"
"Fucking boosted TP dude"
Uh, what are you going to accomplish with this? I fucking realize that landing hooks is better than not landing hooks, you aren't going to magically cause me to be better by telling me to land them. And I made a TP play as best as I could, what's done is done.