In addition to being extremely insulative, wool is ideal for something like a coffee cozy because it is stable in the presence of a fairly high degree of applied heat, it is un-meltable, and it has a very high combustion point.
For a practical example of wool’s ability to withstand heat, when pressing wool, the recommended iron setting is 148 °C (300 °F), which is well above the boiling point of water, which is 100 °C (212 °F).
Measurements determining flame retardance show that wool has an “ignition temperature” of 570-600°C (1058-1112°F), and does not melt at any temperature. (See table below.) This is very favorable in comparison to other fibers, for any application near a non-flame heat source.
So enjoy your coffee cozy with peace of mind!