3 Reasons Not to Stack a Mousepad on a Desk Mat
It usually doesn’t improve performance Modern optical and laser mice track well on most surfaces, including felt. Adding a mousepad on top of a decent desk mat rarely makes tracking better.
It can hurt consistency Stacking layers creates a height difference and can cause the mousepad to slide around on the felt. This makes your aim less reliable.
It is unnecessary clutter for most people Unless you need a specific surface texture for high-level gaming, a single desk mat is all you need. Only add a hard pad if your felt mat is so thick that your mouse sinks in and tracking gets janky.