The (More Than) Complete Cast of CVRPG
Heroes, Villains, Low-lives, and Agents of Leisure
New York is a dangerous place. It's the kind of place where a kind could go into a pet store to pick up four turtles one minute, and end up nearly crushed under a truck filled with barrels of mutagenic waste the next. Sadly, this happened, the kid lost his turtles in the incident, and their bowl rolled off into the sewers... followed promptly by a barrel of the waste.
The mutagen covered the turtles, changing them into humanoid creatures. It also transformed their soon-to-be sensei into a giant, talking rat. He raised the turtles, trained them to be ninjas (because all talking rats know ninjitsu -- it's in their union bylaws), and sent them out into the city to act as silent, shadowy protectors.
While Mikey was great at the ninja, he wasn't so good at the "silent" or "shadowy" parts. The fun-loving one of the group, Mikey was much better at making distractions and finding direct routes through problems. While stealth and subtlety have their place, sometimes what you need is something loud (and amusing) to get through a situation (or, at least, to distract the bad guys while the others work their stealthy magic).
Mikey's addition to the team proves that, for a team to truly work, diversity is required. He's the goofball, to fun one, but also an essential member of the group.
Although often considered the "dumb one", Mikey is actually smart in his own way. Sure he is at times a little distractable, and other times he focuses on things that aren't really important, but he also has a quiet intelligence within him that's sometimes hard to see.
For one, Mikey has an eidetic memory, the ability to see something and reproduce the results without having to think about it. While he may not have the book smarts to be able to work experiments like Donatello, he can watch what Donnie does and redo the exact same thing over and over (useful sometimes in the lab).
He can also naturally work out quick scenarios and find the most direct route through a problem. This is handy in situations where the normal resources the turtles would use (shadows, cover, Donnie's tech) aren't available (like in Dimension X), and suddenly Mikey is in charge, handling the unusual as if it's just another day at the beach.