The films don't really give themselves a need to explain the mentats beyond "they're good at maths".
I do think they could have done better at showing that mentats are capable of huge feats of computation and planning and take the place of advanced computers, and that wouldn't need exposition. The "answer a numerical question with unnecessary decimal places" trope was worn when Commander Data did it for the millionth time. Moreover, it was something that seemed like a simple multiplication: something normal humans who are good at mental arithmetic can do. Having Thufir do the eye thing to deduce the exact location of the hunter-killer agent based on a huge stream of data would have been a good way to do it, for example. That would have made it clearer that Thufir (and by extension Piter via the lip tattoo) was more than a uniformed wedding planner and is actually a powerful, indispensable and dangerously skilled superhuman.
Likewise having someone lament that, say, an ornithopter or carryall could use an autopilot and someone reply "ha, yes, and get the planet nuked from orbit by the Great Families for harbouring a thinking machine, not a good plan" would have shown the approximate limits on technology leading to the need for mentats.
Not showing that didn't really affect the story they did choose tell (i.e. one that, for example, doesn't ever mention or allude to the Butlerian Jihad), but I think they could have added just a little more useful depth without it just being superfluous book details added for the book fans to notice.
One wonders if they left out the war on thinking machines as being at risk of breaking the suspension of disbelief for being too (pre-!)derivative of the Matrix and being overly close to current zeitgeist with LLMs dominating every conversation.