Routinely blocking the air inlets and outlets on a laptop increases the probability that electronic components will fail over time, though most modern laptops have multiple forms over overheat protection and will throttle CPU speed or even shut down if they get too hot. I should note that all the airflow in a Macbook Pro is through the sides and rear, not the bottom, so setting it on a soft surface would not typically block the vents.

The temperatures that start to increase failure rates for electronic components are far lower than those that trigger thermal runaway in Li-ion batteries. The least thermally stable Li-ion batteries, those with cobalt-based cathodes, are at risk of thermal runaway over 150C. I've never heard of poor cooling being the root cause of a laptop battery fire.