Is Cinnamon just the UI? Is it supposed to be lightweight or what? They're lousy at simply saying "this is what this is." Same goes for MATE.
cinnamon is not as lightweight as lxde, ut lighter than kde
Cinnamon is a desktop environment, so it is the UI as well as a large number of applications and tools. My understanding is that it is an effort to keep a more classic styled type of environment rather than going with something ultra modern like Unity (Ubuntu).
It is lighter than Unity and KDE but heavier than Mate. From my experience, if you are running Linux Mint then Cinnamon is the way to go because the bulk of the development is with that particular DE in mind.