Whether or not it's tacky is besides the point.
Mrs. Obama's arms are completely exposed. That short dress also has a deep V neck. It's a very immodest, un-First Lady-like look. Also, notice that the men around her are all wearing ties. Now was it like 300 degrees out that day that she had to expose all that flesh? I'm sure men get hot as well. But if they're working, they still have to put on ties.