You've asked two independent questions: (1) Are sex-selective abortions immoral? (2) Does it make sense to try preventing sex-selective abortions where abortion is generally legal? Now, 'make sense' in (2) can be construed at least two ways: (2a) Is it a practical policy to try preventing sex-selective abortions where abortion is generally legal? (2b) Is it morally consistent to try preventing sex-selective abortions where abortion is generally legal? Question (2a) is a largely empirical question having to do with how effective such a policy would be versus the practical costs of enforcing it. Question (2b) is a philosophical question. One could consistently give different answers to (2a) and (2b).
As for (2b), I think that any legal system that regards abortion as lawful is committed to regarding abortion as not seriously immoral, because I take it to be one of the law's essential functions to outlaw seriously immoral things if there are any. But I also think that if abortion (as such) is immoral, then abortion is seriously immoral: I can't see how abortion could be immoral but only non-seriously immoral. So I conclude that any legal system that regards abortion as lawful is committed to regarding abortion as not immoral at all -- i.e., as a morally neutral medical procedure.
But if abortion is a morally neutral medical procedure, I wonder about the moral consistency of regarding sex-selective abortion as immoral. One might say that (3) sex-selective abortion expresses or reflects contempt for a particular biological sex, but (3) is a basis for questioning only sex-selective abortion of that particular sex rather than questioning sex-selective abortion as such. Furthermore, if the legal system then outlaws sex-selective abortions of one particular sex only, it might thereby express or reflect contempt for the other sex! One might say that (4) sex-selective abortion, as such, reflects a mindset that places too high a priority on someone's biological sex at birth. But (4) seems weak as a basis for regarding sex-selective abortion as immoral in any legal system that regards abortion per se as morally neutral. I'm not sure if (3) is any better as a basis for regarding sex-selective abortion as immoral where abortion is otherwise legal.