Can I tell my team mate to use his brain?
That is mean. What do you plan on saying to him?
He should've used his brain to go for a better position, why did he run straight at the opponent?
If you really feel he made a bad play, then it's good of you to let him know.  You just want him to play the best game he can.  I hope he learns from this mistake!
That's not why I was mad at him. It's because he was stupid. I hate it when people don't use their brains.
That sounds really harsh and overly critical. If you feel like they're not thinking things through why not try to help them do so instead? I'm sure they'd appreciate it and you'd be a better team player for it.