Q: Are Microsoft monthly Update Rollups for Windows 8 and Windows Server 2012 cumulative?
A: Typically, cumulative updates are, as the name states, cumulative. Thus, Cumulative Update 2 would contain everything in Cumulative Update 1.
However, the Microsoft updates for Windows 8 and Windows Server 2012 are called Update Rollups and not Cumulative Updates, and they are NOT cumulative. This means it would be necessary to install the initial general release update rollup, the November 2012 update rollup, the December 2012 update rollup and so on.
This can be seen if you look at all the update rollups currently available for Windows Server 2012. Note the sizes don't always increase from the earliest rollup (KB2756872) to the latest (KB2836988), showing they're not cumulative.