There are ways to offer criticism without being a neanderthal about it.

"You're bad!" Or "Stop being bad!"

Is a neanderthal comment.

Try: "Hey, I noticed that you're not using X in your rotation, if you added it in, it would raise your damage by a pretty good bit."

Or

"I noticed on that last pull you seem to forget to use some of your defensive cooldowns. It would help our healers quite a bit if you could use them more often."

Or

"That enemy does a big hit against anything in its forward arc, even though it doesn't have an AoE warning cone. Make sure to stand off to the side rather than where I'm standing, that'swhy I'm trying to face it away from you guys." (As the tank)

I've used all 3 of those (and no, I don't parse) and haven't gotten any flak from players.