Mia had decided to give her child up long before Nicole ever came on the scene. It wa her choice, and she knew it was best for her baby, that she could never raise a child herself. All she wanted was to go to Japan and have a dancing career.
She changed her mind about Japan, once she was there, and then regretted not having at least seen her baby.
Yes, Mia is young, but she made her choice and there was no going back. The fact is that Nicole DID get Mia in to see Grace, made sure she was able to hold her and say good-bye. Mia loved little Grace, and just because she did not know that was the child that she gave birth to....Nicole did the best she could to insure that Mia was able to hold her baby and say good-bye.
Now yes, I feel for Mia, in that she believes her own baby is all safe & healthy in the DiMera house, and that will be hard to take when the final reveal comes to pass. But most females who give up a baby to be adopted never see that child again, or even know who adopted their baby. At least, not until they are grown, if they are lucky and want to make contact.