It boils down to one thing:
Are you antagonizing your teammate?
If you are purposefully trying to do any of the following, you're automatically lowering your chances of victory.
- Make them feel bad
- Irritate/troll them
- Get them to go afk, quit the game, uninstall LoL, or "just go die"
If you have noble intentions (helping them improve), but poor wording, you can also be a net negative.
A few key ideas when offering criticism in-game to strangers:
a) Use positive messages, such as "When we respawn, can you keep a ward in the tri-brush?" instead of "No ward in tri-brush makes you a bad support."
b) Don't tell them things they already know, such as "You need to stop dying." (if they're 0/6, they already realize that they're feeding).
c) Offer to help (in-game). Things like "you're way behind, and not much use right now. do you want to farm top to try to catch up?" works much better than "you might as well afk, you're useless", or saying "its usually better to gank the winning lane, not try to save a losing lane. but if you want, you can farm my jungle camp for a bit extra gold/xp." is much more positive then "no, you suck".
Of course, this all goes out the window when someone ELSE on your team starts provoking whoever is doing poorly.
In that case, my only advice is to come to the harassed player's defense ("hey, bad games happen, lets stay positive") and encourage the other player to use Mute. Silence implies consent of the bad behavior, and many times simply knowing that the rest of the team are all "cool guys" is enough to keep a player from raging under the abuse of a single negative teammate.