Yes, it is fair for Michelle Obama to give up her career to be mom-in-chief, because Michelle Obama's position was largely ceremonious. Michelle Obama was paid a large amount of money for her work at the hospital. It was only a back handed way for the hospital to pay off a politician.
In some ways, Michelle Obama chose to be "Mom-in-Chief" by agreeing to let Barack Obama run for the presidency. She knew exactly what that would mean for her own career before her husband was even elected. Technically, she could live her own life and go through her own career if she truly wished to do so. She's already creating her own initiatives and programs anyways, though.
People might not be happy in their job but forced to stay there due to necessity - the need for money or legitimacy. Still, everyone has the option to stay or quit in theory. People in the position of the Obamas have all the options in front of them, and they both fully knew where they'd be ahead of time.
Michelle Obama had the choice of whether or not to continue to work at her career. Once her husband became successful, it was her choice to quit. No one forced her to do it. Besides, we could argue that she has been leading a successful "career" inside of the White House, by promoting causes she believes in, such as healthy eating. Being the First Lady is not necessarily the end of Mrs. Obama's career life, and there could very well be more to come, perhaps soon, or perhaps after her girls are grown.
If Michelle Obama was told to cease pursuing any business opportunities that appealed to her this would be unjustified, but if she hates the role she's in, she's a very good actress. This seems to be a voluntary move and because of that there really isn't anything wrong with it.