It is difficult to put Inside Mari into a box to discuss what it's about. It sort of deconstructs the body swap and gender swap genres. It sort of tackles body and gender dysmorphia, and exploring sexuality in that context. It sort of discusses abusive parenting, shallow relationships, and the difficulty of understanding other people.

The thread running through all of that is the question of who Mari is and where she went. And the answer, which I think was both somewhat obvious for several volumes and the most realistic answer, is that she never really went anywhere. She hated her life, hated herself, and she tried to escape through the life of Isao.

I think anyone with an unhealthy dose of self-hate has done this to some small extent. You look at someone closer to the ideal that you want to strive for and you copy them. I got really into running after reading about how Murakami ran every day. Hell, I only really expanded my life beyond video games and books because I read about Miyamoto's inspirations for his games, like Pikmin emerging from his gardening hobby. Mari takes this to an extreme extent, losing her memories of herself in order to fully live as Isao, a man with no obligations, no attachments to anyone, and whose hobbies are simply masturbating and gaming. She doesn't want to change things a bit, she wants to burn the whole thing down and stop existing. Which, honestly, is also relatable.

In the end we see that Mari is Mari. There's no changing that, despite everything. But by ripping apart the pieces of her life that she hated, heedless of the consequences because she wasn't even cognizant of being Mari, she finds herself in a situation that appears to be better. At the very least she has been able to confront and escape the toxicity of her various relationships, which is nice.

Of course, we can only see a brief glimpse of her life from the outside, filtered to what the mangaka wants to show us, and we know that doesn't let us fully understand what's going on inside Mari.