Is the pregnant mother's inability to perform the acts of worship and good deeds she was accustomed to before pregnancy, due to extreme fatigue and morning sickness, considered a sin and a shortcoming in her duty towards Allah?

There is no sin in abandoning recommended acts if the obligatory duties are fulfilled. However, supererogatory (nafl) acts are a great means of drawing closer to Allah and attaining His love, as stated in the Hadith Qudsi: "My servant continues to draw near to Me with supererogatory deeds until I love him." Striving against one's self to perform supererogatory acts is a good thing, and the greater the difficulty, the greater the reward. If illness is what prevents you from continuing with supererogatory acts, then its reward will be written for you as if you were healthy, based on the Prophet, peace and blessings be upon him, saying: "If a servant falls ill or travels, the like of what he used to do when he was healthy and resident will be written for him." So, do what you are able to, and for what you are unable to consistently do due to your health, its reward is written for you.