What specific qualities are considered “undesirable,”
That would be a very long list, even beyond such trivial generalities as “hereditary diseases” and somewhat grayer areas like “stupidity, or unnecessary ugliness”, and so is mostly defined by exception. Specifically, the Citizen Eugenics Board define the alpha baseline for each species, which is the long, long list defining all the minimal acceptable standards for making a new person.
(It bears very little resemblance to the actual species baseline, being augmented out the wazoo, but then, progress happens, and the genius of 792 could easily be the mental cripple of 7921. So it goes.)
Basically, if your design falls below the alpha baseline in any way, you’re in dysgenic territory. You can, however, go sideways as well as upwards: you can, for example, trade the ability to breathe air for the ability to breathe water and still be safe, or swap out most normal integument and senses for different ones in the process of becoming a giant corona-dwelling space whale without trouble. But you can’t reduce your child below the limit unless they gain at least as much back somewhere else suitably appropriate or necessary.
[It doesn’t apply if you’re building new bodies for the market, note: if you want to, or your customer wants to, informedly download themselves into an enfeebled moron, well, that’s their dumb-assed free choice. But you can’t inflict that on dependents.]
and does eldraeic law have any recourse to remedies for a dysgenically reproduced sophont whose existence is a fait accompli?
Well, they could sue their parent(s) for eye-bleeding amounts of weregeld and compensation, but the odds are the CEB have already done that on their behalf long before it would ever occur to them. It comes to the same thing in the end.