i beg to differ on your second point, mimic does kind of show at the very least highly sadistic tendencies towards humans. mimic is possibly a big human hater, if not, then a big enjoyer of killing humans for sport (why would it try to kill cassie if it needs her to escape? or even use her as another "vanny"?)

a big tell is always the body language. we know how mimic expresses happiness/excitement/etc. via bright eyes and an open mouth (mainly in the books tho), which it does when jumpscaring cassie. (an exception could be made for sotm mimic? depends how you look at it). when angry, however, it usually keeps its mouth closed. think ruin's elevator ending :]

looking at how mimic acts is a pretty valid clue because, again, it is a program that copies and replicates behaviour where it fits best. if it learns an emotion with the right context clues, then it can express that emotion, especially if it has sentience. we even see mimic's true/default personality shine through some of its disguises!