installation does apply to things other than Ubuntu as a whole (I can install a program, which would be within the scope of that tag ...I can install more memory in my computer, which might not be within that scope.) But so can setup. I doubt we gain anything by having these as separate tags.
As a separate point, as Seth says, setup seems to refer to an even wider range of unrelated topics than installation, with less clarity as to what falls in its scope. I can set up a new user account, a second monitor, document templates in LibreOffice, etc.
I don't think we need setup. It's too ambiguous.
In my opinion, setup comes after installation (installation->setup). You usually install once and able to setup configurations several times.