Yes, the mother of Trayvon Martin should have ushered in the Hilary Clinton nomination. Currently, there is a great deal of divisiveness in this country concerning race and race relations. One thing that will help with this is for people to see different groups working together and getting along with each other.
Yes, it's perfectly fine for Trayvon Martin's mother to have been at the DNC ushering in Hilary Clinton's nomination, so long as she was not forced or coerced to do so. If she was there supporting Clinton of her own free will, I see nothing wrong with her doing so.
If Hillary really cared about Travon Martin, she would immediately pledge to end the war on drugs that puts the police in a position of antagonism against black youth. She's only using the optics of the story to help her political career, same as always. She's the most powerful woman in the United States, she could speak to the underlying problems but never does.
Although the mother of a black individual killed by police may be significant to the black rights movement on some level, she certainly has no accreditation that makes her a valuable contributor to the convention. A leader of an organization such as the NAACP or Black Live Matter group would have been a more sensible choice.