WARNING: PLENTY OF SPOILERS! YOU'VE BEEN WARNED, BUB!!
I'd give this movie 3.5 out of 5 stars, but this is the only way people will read my review.
If you walk into the theater expecting to see the old "canon" TV shows you grew up with, you will be EXTREMELY DISAPPOINTED! Teenage Mutant Ninja Turtles: Mutant Mayhem is everything you think you know about the original TMNT stories backwards.
The movie explores the theme of acceptance. The turtles are viewed as monsters and freaks. But when a crime wave led by Superfly and his goons breaks out, the turtles, with the help of their human friend, April O'Neil, try to defeat Superfly to become accepted and portrayed as heroes.
The Turtles were kinda flat characters who didn't change throughout the movie.
Leonardo thinks he is a leader, but he's not, and he breaks down every time someone tells him he's not a leader. Another problem with Leo is that he has a crush on April O'Neil instead of Michelangelo, which ruins everything.
Raphael is known for being the rage-monster of the TMNT, and he plays to that strength in the movie, which is okay but I wish we got to know him a bit more, like why he's angry all the time.
I don't know if I have anything positive to say about Donatello. He's always been my favorite turtle and my favorite color, but now he likes anime. Gross. Also, he complains about how he has the worst weapon of them all: The Bo Staff, which is essentially a long stick. His master plan to defeat Superfly was inspired by an episode of "Attack On Titan."
Michelangelo was great though, unlike how he was in the PG-13 TMNTs. He was the funniest, the weirdest, the most creative, and he's got them mad skills.
The jokes made me laugh at first, specifically Splinter's fear of his sons getting milked on the surface, but after a while, they started to repeat themselves. April O'Neil throwing up on live TV made me gag and wasn't funny at all.
Superfly is a weird choice for a main villain, but he's super cool and works anyway. Those extra, useless arms dangling out of his stomach were creepy though. He drops way too much language for a kids movie, cursing enough to make your kids ask "Daddy, what did those words mean?" on the car ride home.
Supporting villains like Bebop, Rocksteady, and Mondo Gecko were goofy and brought back plenty of nostalgia, but they were just henchmen to Superfly who had no mind of their own until the very end of the movie.
Why the heck are these legendary villains being reduced to mindless henchmen?!
Shredder was missed and mourned, but a post credit scene suggests that there will be a sequel, with The Shredder as the villain.
To sum everything up: If you're comparing this movie to every movie ever made, it's just okay.
But if you're comparing this movie to other TMNT movies, this is the best TMNT movie so far.