It could be simply that mimicry only goes so far. I disagree with those who claim that mimics would have differently colored blood, smell like corpses, be unusually heavy, etc. See, these things have evolved to mimic people. That means they have evolved to infiltrate human society, and considering the complexity of a leaf insect or octopus' camouflage, it's quite likely these things can perfectly mimic the human body, at least on the outside.
However, it's what on the inside that counts, or in other words, the mind and heart. Let's say the mimic takes out and takes the place of an artisan. There are two big problems with this the mimic can't overcome: A) it lacks the artisan's creative vision and B) it lacks his/her passion. It also doesn't know the thousand little things that go into everyday social interaction: inside jokes, what relationships the victim has, how he/she behaves around different people (because we do, generally, become different people depending on who we're with).....the list goes on and on.
If you want your mimics to be a viable threat, you need to give them the four following traits:
- Intelligence (must be cunning to evade and/or negate suspicion)
- Intuitive (must be perceptive, have highly honed social instincts-in other words, it needs the sociopath's abilities to understand other people despite having messed-up instincts)
- (may neutralize 2, but unlikely) Power of Inheritance: Once a mimic ingests a victim, their magic-infused digestive system converts the victim's DNA into a form the mimic can absorb and turn into another form. However, there are drawbacks to this:
a. Genetic Memory-While this enables the Mimic to know all the important details that allow them to pull off this trickery, it also gives them the meaning that goes with those memories. A normal Mimic may not access this meaning, as it's brain does not have the right structure to process it, but a Mimic with a recessive gene will. In other words, it will carry all the sentiments that artisan collected over his lifetime.
b. Genetics has Little to Do With Personality-One of the arguments against cloning pets is that personality isn't determined purely by genetics. You may have an angelic bulldog for a pet, but his clone may be temperamental and just plain mean. In other words, the Mimic will most likely still lack the creative style or idiosyncrasies of their victim, a potentially fatal flaw.
For evidence of the above statement on genetics, please check out this link:
https://www.fda.gov/animal-veterinary/animal-cloning/myths-about-cloning#:~:text=to%20the%20top-,Myth%3A%20Clones%20have%20exactly%20the%20same%20temperament%20and%20personality%20as,%E2%80%9Cnature%20versus%20nurture%E2%80%9D%20argument.If it doesn't work, please let me know so I can fix it.
Finally, an alternative method is that the Mimic somehow 'absorbs' their victim, integrating their very being into their own. This of course comes with problems of its own-what if the victim manages to retain some control, or takes back their new body? If the Mimic is killed, would the victim's spirit take over the body and simply resume life as best they can?
This may result in a Gothic/medieval version of Among Us, where the 'crewmates' (AKA villagers) would be trying to root out and destroy the shapeshifting aliens (AKA monsters, Mimics).