When viewed in the consumer product marketing context, the MM was a marketplace success.
The MM was, in marketing terms a "line extension". This occurs when a company takes a product design that is well-established and 'mature', and tweaks its features and benefits to appeal to a different buyer, in order to increase sales and extend the product's life span.
The MM was cheap to develop, and make. The Panther design is old and well established. Most of the special MM mechanical parts were borrowed from other product lines. So, with some upgraded trim, and fancy gauges, you extend the Panther sales and appeal to a buyer who probably would not buy a Marquis or Crown Vic.
IMO they didn't have to sell many of them, or for very long, for the MM project to be successful and profitable.