The Scrappy: The threeRaving Rabbids are hated for being very out of place and nonsensical even in the context of the publisher, their brand of humor not fitting the game, and wasting slots that could have gone to well-known TMNT characters.
So Okay, It's Average: Some people defend it as a decent fighter (especially good for a licensed game) while others attack the small roster and lack of major TMNT fanservice, but the general consensus is that it's this. Disappointing, considering it was made by one of the same devs who made the critically praised Brawl.
The Problem with Licensed Games: The game has very little content. A story mode that lasts a whole 30 minutes, a repetitive survival mode, and 51 extremely short honestly-not-stolen-from-Super-Smash-Brothers-Missions. That's it.
They Just Didn't Care: Many fans feel the devs simply didn't care about the game; despite being released to mark the 25th Anniversary of the series, the game only has 13 characters from the series... and then 3 characters from Ubisoft's own franchises. The 1980's series (you know; the single most famous thing about the turtles) is not even represented aside from a commercial in the special features and the Technodrome being the background of a level (the level you can't unlock without using a cheat code).
It's not hard to find lists online from angry fans of valid TMNT characters that were left out of the roster in favor of Rabbids. One such list on a Ubisoft forum had 150.
One of the characters and one of the levels are physically impossible to unlock without cheating.