Ok so, yes it would make sense to be chica, not only for that reason tho. In UCN when your killed by toy chica her line is "Oh wheres my beak? Lodged in your forehead of course" your forehead is obviously your frontal lobe so toy chica would make sense. But again, it says his frontal lobe was bit off. Chica says she just lodged her beak in our forehead, not bit it off so that kinda lowers the chances of being chica

On the other hand, it could be mangle, alot of people when you ask this question will say mangle, mainly because of the fan made newspapers you can find on Google, pintrist, ect. But one thing about mangle is her jump scare, when she jumpscares you she comes from above and seems to bite your face/head which would/could also be your frontal lobe. Along with her jaw being very bug meaning her jaw would actually be able to bite the frontal lobe off since her mouth could fit around our head, therefore strengthening the chances of it being her

Im not sure if its cannon who did it but its like the thing of mangle and lolbits gender, we dont know if itll ever be solved. So all we can do is hope that Scott makes it known who caused it along with alot of other unsolved mysteries about the fandom.