First off, if you are in low Elo, silver or lower, you are probably incapable of effectively assessing your teammates decisions regardless of whether they are mistakes or good plays. Sure if you are a good player, a plat smurf or something, then yes, you might be able to apply your game knowledge and experience to say "that was a mistake."
Gold players make mistakes frequently, but typically they are still not even that good at figuring out, at the time, that it was a mistake. Mistakes have to be obvious to them or they were the ones making them, or they have a high familiarity with a champion an ally is using to let them know that they are making poor choices or better yet a simple suggestion for the next engage.
The player playing the champion has more knowledge than you, at the given time and is likely more experienced with that champion. Their skills may be on cooldown, they know their champion can't tank specific amounts of damage, they are aware they have no magic resist and won't engage the apc, etc.
The point is that pointing out others mistakes as low elo player yourself should be avoided because:
1. You are not a good player yourself to figure out a mistake
2. You are not playing their champion and probably have a lack of familiarity with said champion
3. It will lead to arguments
What you should do is play around your teammates when you can if they make mistakes and simply work on improving your OWN game verse trying to point out the "flaws" in theirs.
I offer advice to players that I play with that are friends that I know are not quite as good as me. But they are much lower than me, bronze or silver, so their mistakes are typically painfully obvious to me.
When I play among peers, I typically do not even offer advice unless it is very very very clear and they themselves know better (then it is redundant because they are self-aware of their mistake) or they request it.