Is DC-to-DC (24 V to 12 V) buck conversion typically more efficient than AC-to-DC (110 V to 12 V) conversion?
No general statements can be given.

Generally, the larger the input/output voltage ratio is, the smaller the efficiency for basically all converter topologies. 

However, whether you end up with 70%, 85% or 99% efficiency depends on the actual converter and the load.

The reason installation wiring uses higher voltages like 120V or 230V is that the energy losses during transportation will be lower if you need less current to deliver the same power – so, the very standard way of having 120V to racks of equipment and then individual converters where you need your power makes a lot of sense.

AC/DC converters are technically so close to DC/DC converters that the difference between those is shadowed by other technical considerations, such as how much you need to "overdimension" your power supply, how much money you want to invest in copper, and whether you're going for constrained size etc.