Mailers as a mono-action to lead creation is ridiculous, and this form of marketing has never been designed for such any more than a billboard is for getting "leads".
Mailers is more-so a conditioning action.
Could one get a call from "a" mailer, sure, but that's not the consumer psychology of it.
Back in the day, all these tenant's of such were just "known", and followed, because we never had the internet to rely upon. So we would strategically hit a location with a mailer, that than the call-room would start hitting on 2 days later. And as call center worked the area, 2-3 additional mailers went out.
Cold-calling is all but a dead-art, but the principles remain.
Using mailers, in an ascending manner, in concert with conversion based marketing, that's what drives up lead gen to hit those 3,4% marks.
Your mailers have to be conditioning based, referencing back to the converting portion (website / click funnel). And done in strategic waves. Not 1 but 3, 4 etc..
And when done in concert with other mediums, be it billboards, radio, whatever creating a saturation marketing strategy, that's how leads are made.
But problem is too many think they can do just 1 action of the marketing strategy and like magic leads appear, that's gonna be a setup for failure.
So if viewing mailers as the marketing strategy, your doing it wrong. It's a piece of the puzzle, 1 piece, so a person needs that whole to make that 1 piece most effective.
In general, a click-funnel, a call center # (because needs to be answered 24-7), an ascending series to develop a lead in that click-funnel, and a series of mailers that "stack-up" on the messaging of each other.