If offensive language is reportable, playing Rengar should be as well

Technically it is. You can report for whatever you want. It's just that the vast majority of players won't report him just for playing Rengar, so the system will think that your report was false and the dude isn't doing anything wrong.

Nah more of Teemo but sure.

But playing Rengar is just playing the game. There's no mean intent, just intent of trying to win.