I know, it sounds like I'm joking. However, I am dead serious about this post. Mitama is not a good unit at all. And here's why.
Let me clarify my statement first. She is not bad in the way you're thinking. Yeah, she's great at what she does, and she is a HARD carry through the entire game. She's basically a cheat card of a unit. And that's where I believe she falls apart.
I saw a post a little while ago asking if mitama was a fun unit to play with, and it seems the general consensus is that no, she isn't fun to use. Her stats and abilities, albeit very powerful, are just unfun to use. There's hardly risk, because she's got ludicrous survivability. Her DPS is high enough that, when paired with her slow, makes it impossible for enemies to push in, and she can kill them where they stand. Her LD and range are good enough that you almost never have to think about bringing her to a stage.
Sometimes, you have to remember that you're playing a game. Games are supposed to be fun and entertaining. Bringing such a good unit to every battle without needing to think and wiping the stage anyways is not fun. (at least for me. Maybe some of you like to meatshield for a few minutes, with the enemies unable to push, slaughtering everything with a single unit) Challenge is what makes games fun. Trying over and over, perfecting your strategy, and beating a difficult stage to get that hit of dopamine and sense of success is why I started playing battle cats. And that's why I think mitama is a bad unit- she's so good she removes the fun of the game.
Of course, this is just my opinion. You're free to disagree. But i know that if I ever get mitama, she will likely stay level 1 (or maybe i'll level her up and use her as a get out of jail free card).