That's not what mental acuity means.
Mental acuity is intelligence, not any form of psychological manipulation and while some Mentalists may possess it, it is not a requirement or in any way part of the definition of the word.
By virtue of his profession, Patrick Jane is portrayed as having uncanny powers of perception, being able to accurately determine truths about people from casual conversation. However, actual stage mentalist, while perhaps perceptive, rely heavily on tricks, such as researching subjects beforehand and ignoring false psychic "hits", that would not be applicable or useful in the context of crime fighting.