Will a desk mat cause uneven wear on my mousepad?

No, not really. Unless your desk mat has a weird rough texture or your mousepad is paper-thin, the desk mat acts as a neutral base — wear comes from mouse feet, wrist, and time, not the surface underneath. If anything, the desk mat adds cushion and prevents sliding, which can reduce uneven wear from creasing. The only rare exception is if the mat has raised edges or a textured pattern that creates high spots after months of pressure.