Partially right
If you take a look at identical twins, they share the same DNA and look almost similar. But if you look close enough you will notice some minute differences and therefore people close to twins can generally differentiate between them. These differences are due to differences in growing up.
But even if she is not an exact clone, if you share a lot of DNA, you will look similar, Example - Parents and their children, siblings.
So Mallory and MC's mom should at least look similar.
AFAIK, cloning is a genetic manipulation of a already fertilized egg, that it's mitochondrial DNA removed and replaced by the "cloned" DNA. Although sharing 99.7% of the analyzed DNA (it is exceedingly rare to analyze ALL the DNA) it may give the impression of body doubling, but it's not in fact the case.
The predisposition to (DNA based) is quite less strong than learned behavior (Experience based), unless you're talking of morphology.
I am quite interested in how the authors will deal with the Mallory conundrum. Being a work of fiction, all avenues are open, even a "Test Error".
In "moral" terms, there's really nothing interfering on a relationship with the MC, but, "genetically" that's a whole can of worms, as the MC shares a significant portion of DNA with Mallory (close to 50%) that can cause severe reinforcement of bad genes on their offspring, the operative word is "CAN" it's not a absolute certainty it will, and the most severe cases can be detected quite early, or, even cause abrupt termination. There are many cases of "incestuous" babies with this much shared DNA with no problems, although it is NOT a desirable outcome (genetically speaking ONLY) because the reinforced traits will be spread to further generations (BTW that's how animal breeding works, also plants genetic manipulation).
So, go read some genetic science, some medical, or, leave it up to the authors to deal (somehow) with this interesting proposition.
I just hope I don't have to choose between Jaye and Mallory anytime soon... or ever. I know... I know... ::deep sigh::