Don't think it needs to exist.

The asininity level of League far outstrips that of a bad WoW battleground, much less the couple of less than stellar players you get from time to time in LFD. Probably a third of LFD and LFR get carried, but the WoW community doesn't feel the necessity to bash them into oblivion for getting so, the way a goodly number of LoL players do. Nor do as many people feel the need to actively harm their own team, and rage-quitting has a significantly lower impact in a system that auto-refills.

Blizzard's own report function is unneeded enough outside of gold spammers that it can handle the issues, without needing a large playerbase team to help them out. Also, even in LoL, sucking at the game is not a punishable offense. You may want a tribunal system to weed out all the people you think are "bad," or at least keep them away from you. But no system Blizzard creates would do so, and you must remember that LFD is designed to pair you such that you can carry or be carried. Rare is the equa-skill group.