It basically means that the person is attractive or beautiful. The male Peacock are beautiful birds with long feathers in brilliant colorful hues that they use to attracts the female peacock. Therefore if you say you’re as pretty as a peacock you are saying that person is very pretty or handsome.
On my 2nd visit to the Southwick Zoo I captured images of two peacocks interacting as they strutted around showing their mate how beautiful they are. It’s quite a spectacle so I photographed the displaying male in all his glory. I’ve already gotten used to shooting through the openings in the chain link fences around many of the exhibits.
The bottom photo shows the jealous female staring at me saying “Don’t mess with my boyfriend”! LOL