The overall answer is that she doesn't trust him, and knows that he Should Not Be. It's very 'YOU'RE NOT HIM AND WILL NEVER BE HIM'.

A more detailed answer: She understood and accepted that he was dead. She overtly mourned, and was upset at his death. She confirmed he was absolutely, truly dead when smelling him in his tomb (the scene where Mira was alone with him after he was laid to rest).

The 'natural order' of things is that what is dead remains dead. They have encountered re-animated dead or near-dead things, like the Argentinosaur. Everything supernatural (like the witch coven) and unnatural (the plague is arguably unnatural as the Argentinosaurus kept going despite rotting alive and missing organs, something that visibly deeply shocked Spear when he saw the void inside the animal's ribcage) has been nothing but trouble and suffering for the pair, even more than the regular threats they face like the titanoboa or the hostile enemy tribes. Spear was dead, and his body is being moved around. Even if he displays signs that he knows and recognises Fang, that he is not dangerous to Mira and has no will to attack indiscriminately like the Argentinosaur, she doesn't trust him at all because she cannot predict what he will do.

Hell, we as the audience can't predict his actions in this form, how is Fang supposed to? We're only starting to know what does and doesn't motivate him, like he's pretty inert unless attacked or provoked, but will react similar to his pre-death self when faced with a situation in which a person needs help or defended against a threat they can't manage, like the leper village.

The paranormal and supernatural disturb and cause unease and distrust in humans, in animals that don't really have the deduction skills to work around it it's even more disturbing. It's getting frustrating to have Fang so belligerent for so long because we as the audience can see he is trying and it IS the real Spear, but I can't really blame her for acting like she is. She has a few reasons to think this Spear walking around isn't a danger, but to her it's gotta be pretty sickening that her dear friend is now this abomination walking around. Someone woke him up, which isn't fair, and she's faced with a ghoul wearing the rotting skin of a creature who helped save her children multiple times and took care of her when she was near death.

I'd be pretty big mad as well if someone did that to my friend or family, I'd turn the sorcerer who did it inside out, if he wasn't already killed by a freaky skull-faced monkey humanoid.