Are red giants dimmer than when they were main sequence stars?
Red giants are actually brighter than main sequence stars.
A red giant is a smaller star of less than about 8 solar masses which has used up the Hydrogen in its core. The star expands to many times its original radius. It is actually fusing Hydrogen in a shell around the core.
Red giant stars have a cooler surface temperature which makes them redder in colour.
Although cooler, the large size of a red giant makes it brighter overall. It can be thousands of times brighter than the Sun.