Genetic determinism in Star Trek is bad, because the species are allegorical stand-ins for the diversity of mankind. So if you're arguing that they differ on a fundamental way in terms of behavior or aptitude due to DNA, you're basically telling an allegory about the black/white IQ gap, even if you don't realize it.
It's also bad from a storytelling perspective, as we naturally want stories where people behave in a certain way due to their experiences, not just due to something inherent to them. Like, would DS9 have been better if instead of giving Garak his backstory, they just waved away his behavior by saying "Oh, Cardassians are just biologically predisposed to be distrustful?"