UAC is quite different from su / sudo.
Windows NT has always supports the notion of "root" level (aka "Administrator") accounts and standard or limited user accounts. It has also long supported "runas" - the equivalent of sudo. The purpose of that is to allow a standard user to run a program in the context of another user, generally an Administrator, on the same desktop.
UAC, on the other hand, could be called the opposite of "sudo." Instead of running specific processes as a more privileged user, it allows an Administrator to run processes as a LESS privileged user, with varying privilege levels. Technically, Windows has also supported something like this in the past via Discretionary Access Control mechanisms and custom security tokens. UAC brings several additional pieces to the table such as: Mandatory Access Control, more direct user/system control over this behavior, and various bits of supporting infrastructure to make it both more secure (i.e. UIPI) and more compatible with existing programs (File System and Registry virtualization, for example).
UAC also allows programs such as IE and Chrome to run at below-standard privilege levels ("protected mode" or "sandbox" mode), enables secure consent prompts for elevation (more convenient and often more secure versus credential prompts which are vulnerable to spoofing attacks), and more.
So no, UAC is not a ripoff of sudo.