Tbh when I first saw her I initially thought that but now I'm not too sure. Maybe she had been mutated to become the way she now is

Maybe she is a half-human and half-cyborg?

Or is she a fake human because she is actually a cyborg that appears like a human?

Yeah I do. In one of my discussions posted before I believe she is actually Fionna who is Finn's mother but received brain damage when fighting the Lub Glubs causing her to become Susan Strong. The Hyoomans may have some knowledge of technology so in order for Susan to survive, they gave her a transplant.

Susan isn't a Fish person because they finally revealed it in the episode, "Dark Purple"! Looks like she's part human and cyborg or robot or android.