Whenever I hear this argument they say she does not sound smart. They of course are referring to her dialect. They should realize it is ones actions that should be the basis of measuring intelligence.
Speaking (and writing) is an action. There's whole field of psychological study that measures an individual's locutions or writings and estimate a person's intellect from that. Think about it - the SAT's measure mathematics, critical reading and writing.
Language skills, to include the way someone speaks - their grammar, syntax and their breadth of vocabulary reflects how they think and their ability to effectively communicate those thoughts, all reflective of their intellect.
Some of the greatest minds of the last 500 years - Shakespeare, Jefferson, Lincoln (to name a few) are author master's of language.