As zPOOPz and Zombie Gerbil have said, having a report feature in-game would detract pretty heavily from the actual gameplay. (not to mention it'd probably result in overall decreased accuracy of intentional feeding reports, since people can be surprisingly trigger-happy in the midst of a match gone sour.)
I've had to report a few people in Heroes of the Storm, a MOBA that does allow in-game reporting, but the issue is every time I go to report the player, I have to open chat to make their recent offending line visible so I know exactly what to report them for, then hold TAB to open the scoreboard, then click report, click the appropriate option, and then write a short note about why I'm reporting them.
In actuality, it may only be 20~ seconds, but those 20~ seconds make all the difference in-game.
Beyond that, how would in-game reporting help to punish trolls/intentional feeders? The procedure is still the same - the match has to end for the behavior to be reviewed, and it is wholly possible that the person being reported was not intentionally feeding or trolling.