Nobody should be immune to criticism, Everybody can do better at something and criticism can push you to become better at something. Just because she is the first lady she can also make mistakes, and therefore should not be immune to criticism or anybody correcting her. Even if it was illegal how would you catch the people? They would just criticize her in public.
Although open criticism of the first lady should be made with respect, an exemption is not necessary. Every public figure and dignitary will be scrutinized by the public. Unfortunately, being married to the president makes you a public figure by default. Having an opinion is a human's right and should be freely expressed. Criticism is a form of voicing one's opinion.
Trump's remarks were not necessarily accurate. Michelle Obama has the right to campaing. I don't know why he critized her or what is wrong with his brain, but even so that does not mean that First Ladies should be immune from criticism. The two ideas have nothing to do with each other.
No, First Ladies should not be immune to criticism. These Ladies stand beside the President as the elected leaders of the United States. If the First Ladies did not accept criticism, I feel it would be a lack of good sportsmanship during the debates. Although slander is highly uncalled for, speaking critical truths of anyone directly involved with the candidate is acceptable.
First Ladies are public figures and often play an active role in politics. Therefore it is perfectly reasonable for those who disagree with them to openly express their views. In an open and democratic society, anyone who expresses a political opinion should be subject to robust challenge if it is deemed necessary as this preserves the principles of open debate.