Is Mimi a bad character or am I just stupid enough to don't get how to play well with her?

[Cicilka]

The other characters have combinations of perks, itens, backgrounds and/or game mechanics that they benefit from, but Mimi's perks are, in my experience, trash. Also, the scalpel doesn't seem to be valuable enough to compensate Mimi's Obsession. Is she supposed to be a more "challenging" character, since she is unlocked after beating the game for the first time, plainly bad or there is something I'm missing?

[Ryōshū]

Mimi is a great character, scalpel is a decent starting weapon that takes advantage of her KNW stat and her downside can be played around. She really shines on the hardest difficulty (True Believer) where you start with 0 funds and therefore will have more difficulty getting a decent weapon to start.

As for her unique perks, let's take a look at them:

Human Anatomy - +1 Damage against Human enemies is pretty decent, especially since this game has a lot of them and even more with the "Hunted by the Cult" status. Bosses like the Terrible Old Man, Mad Janitor, Aka Manto when you take the cloth, and Takashi san in the lighthouse are also human.

Fast Learner - Gives 3 or 4 extra EXP if you get damaged in combat (not a lot). Definitely her weakest perk but it synergizes well with her last and most powerful perk.

First Aid - +2 Sanity after combat is HUGE. Easily her best perk and one of the best perks in the game and essentially solves her sanity problem despite starting with Mimi's Obsession. Combined with Fast Learner you can shrug off combat damage and get extra exp. It also makes her a potent spellcaster too since most spells cost sanity.

So yea, essentially I think you overlooked how good the First Aid perk is and how it more than compensates for her starting downside of -20% max sanity. Mimi is also the only character to start with a solid combat weapon. Moriko starts with a shotgun but it is underwhelming without bullets or her perk that strengthens her firearm melee speed.

[Griffith]

....But what IS Mimi's Obsession?

[Cicilka]

Mimi's Obsession is her starting perk, it has an efffect of -20% max sanity.

[Ryōshū]

As for what Mimi is obsessed with? That remains a mystery...

[basketofseals]

Cutting people up seems like a common headcanon.

[rygar8bit]

Think she's second best after Mizuki. Knowledge weapons seem to be the best.