So after the Ralsei told us about the underworld being fake does it make undertale world fake too , cause Undertale was in a mountain/cave where there was no light, so it means it all was main character imagination?
no
It's not "the underworld", that was never a thing, it's dark worlds in Deltarune and the underground in Undertale
Light World:Some amount of light, ANY AT ALL.
The Underground would have still been in the Light World, if it is the same continuity as Deltarune (it’s a different universe though. The main connection between the two worlds is Sans. (and maybe Gaster)
Dark World: Literally a negative amount of light (What if you could take away the light that wasn’t there?)