I'd avoid putting anything on top of your monitors. The cabinets are designed to resonate, and if the manufacturer wants them to resonate less, they build them thicker and with denser materials.
But it is super important to decouple monitors from your desk. When they sit directly on the desk (or another piece of furniture), the desk vibrates in sympathy with the monitors and inadvertently becomes its own speaker, adding small amounts of bass and low mids. It mucks things up, making the music sound mushier than it should. MoPads help. Putting your monitors on dedicated stands behind your desk helps even more. Even cheap stands from Amazon. Things just sound clearer.