I used to hate myself, like a lot lot. Now just a lot. HAHAHA. I love and hate myself at the same time so yes the doesn't matter front is indeed from experience. I know i'm super critical about myself hence i hate me. But yeah sometimes i just don't care, maybe a form of defense mechanism to cope up with the stress or i wanna change and like myself even just a bit, maybe Niimi is trying to project this too.
And him not being able to accept sawamura's feelings (when he ask him to date him) at first is also a sign of him not liking himself. But Niimi our brave lil chick.
I'll admit, this manga captures self-loathing and shame to a T. Niimi being so over critical of himself and constantly feeling less worthy........but putting on such a bold/ not affected/ doesn't matter front.....this author's probably writing from experience. I go through this phase on and off, and it aint fun.