I've heard that Lubuntu consumes less resources than Ubuntu. Is it because of the fact that Lubuntu uses different desktop environment and window manager?
If so, would it be equivalent to install lubuntu-desktop on top of Ubuntu rather than doing a fresh install?
Exactly.
Lubuntu is essentially Ubuntu, but with a different window manager / graphical shell (LXDE) and a different selection of default applications. For example, the default word processing tool is the lightweight, but powerful abiword rather than LibreOffice Writer, gnumeric instead of LibreOffice Calc, audacious for playing music etc.
You can achieve most of that just by changing the selection of installed software and selecting a session type from the login screen, but installing lubuntu-desktop will simplify this task. Moreover, as far as I understand it, installing lubuntu-desktop will not break your regular Ubuntu installation, you will be able to either use your regular Ubuntu desktop as well or remove lubuntu completely.
