You know the justification for why MC didn't notice anything 'weird or strange'...The bullied MC is not paying attention to her surroundings...She would see all the blood hand marks on the windows when walking to school but she is mostly just staring at the ground while walking...You can notice that MC not even looking at the girl who saved her from bullying face, her gaze always pointed down (Meek, defeated and submissive...Submissive and breed@ble mode basically). It happens to bullied kids that are low on mental energy, have no time to look around and admire their surrounds due to all the negative emotions they are dealing...Even sad and angry people will notice that their food taste worse or more dull due to their negative emotions.
Though I am surprised that the zombie(Guy walking past her) didn't try to attack her...Perhaps there is a element from like world war Z that being sick (Minor cold) or having Cancer makes you invisible to zombies?
Yes, MC's reaction and what she admit does sound crazy and will set off the alarm bells in us normal people...But eh I don't blame her...She had a lot of built up negative emotions waiting to burst out. Risa is kind of worried and scared when MC told her that since killing other zombie is not personal for her(Teleports behind zombie with bat and says "nothing personal kid"), it's just for survival. MC on the other hand...It very personally~!
Like some comments said and point out...There might be a cure or vaccine being made during the 1 to 2-weeks that MC been sick during the zombie outbreak [Shaun of the dead after outbreak style]. Serum or possible serum?
Donkey Kong comment reminds me of the [All of Us Are Dead] Korean zombie movie, like how some people if their willpower or determination is strong enough then they won't become mindless zombie due to the virus (Some examples shown in movie).
MC's new friend/best friendo (Future waifu)~! is immune or has a zombie resistance [Half zombie?]. I don't believe the MC reaction and emotions...if you had the chance to hurt or bash the heads of the bullies that been making fun and tormenting you for almost everyday then MC's reaction is very understandable and reasonable...'Normal people' would be disturbed and a bit scared though...
MC showing off her inherent yandere-traits to her new best friend (Possibly her first ever friend...)...MC please don't kill your friend's other friend, they might be dating!...A 3some is the best solution if they are though...though if that doesn't work then you can always 'wish or hope' for a possible unfortunate 'accident' to happen to Miya tho...Comforting your new best friend XD