The ego is a fragile thing. Most who sing karaoke on a regular basis have slightly larger egos and therefore take a harder fall when judged and/or critiqued negatively. There are many google searches that teach proper mic technique, voice control, etc. Now, if you were to set up a teaching clinic on the various ways to improve your performance then perhaps you'd have a chance.

People who are out with their friends having cocktails want to have fun. Having some food and singing a few songs are the reason they are there. Being randomly critiqued on how you sing, stage presence, etc. sounds like a recipe for disaster.

Personally, I would not enjoy being told I was not that great or that I need to pick a different song. It takes guts for some to even consider singing in front of other people and telling them they suck seems inappropriate and not conducive for you as the host keeping them coming back.

I love the idea of a Gong show. Having a talent show where anyone can get up and do anything is far different than being critiqued on a karaoke performance.