To note that this is because people needed a word to describe someone who's "utterly mental" but didn't like "crazy" or "insane" (which are both considered not-PC these days, among other things), so they reached for the closest they'd heard - mentalist, made famous by the TV series - even though it was not the correct word.

But yeah, it is now in common usage. A bit like their/they're/there and "should of", it hurts my pedantic sensibilities, but in the end, people will speak how they want to speak and if enough of them agree, the language just changes.