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Remember Mikey's touching speech in Samurai Tourist?
It's strikes me that it's not just Leo that does this - taking on a role in the family so the others are free to do their own things. I'd argue they all do it.
Donnie almost entirely shoulders the maintenance and security of their lair (though I like to headcanon that the others help out with chores. We see Leo doing some welding in Tale of Master Yoshi, for example). This frees up his brothers to enjoy their hobbies - Mikey to read his comic books and play video games, Raph to tinker with his bike and hang out with Casey, and Leo to read and meditate.
Raph strikes me as the Guard or Shield of the family. Especially on missions, he's the one that watches their backs and keeps them all safe. Which leaves Leo free to focus on the strategy and overall logistics, Don free to let his big brain tick over the science and Intel they need, and Mikey free to bring some levity to the situation so the stress doesn't get on top of them.
And Mikey? I'd argue that Mikey's intentionally the goofball of the family. He's far from oblivious, as the above quote shows. I think he deliberately adopted the persona of the 'clown', in order to make his family smile and relieve the tension that having 3 very intense brothers tends to foster. Mikey's busy being the 'funny one', so his brothers don't become snowed under with their own heavy thoughts and self reflections.