It might not sound like it but it actually is quite helpful - translated it means "you are not doing well at a very basic thing and need more practice" because if a guitarist, on stage, can't hit a G-C chord switch competently, then there's really not a lot of wiggle room. They...suck...and need to be shown ways to improve. A lot of times the self-esteem-based response will dismiss the criticism as 'being mean' or some crap when it's actually totally valid.
Probably not - you're supposed to hone your craft through practice before you perform in the way that was described.
If the overall performance was poor, and something so basic was clearly an issue, then maybe they don't realise?
This is like a development candidate being poor at simple interview problems - it simply is the case that often you don't know what you're doing wrong until you've developed skill, but people around you saying you're doing great can suppress your desire to work on that skill in favor of rushing ahead to interview where you fall flat on your face.
(An Open Mic D-Chord is a D-Chord (D A D F#) where they don't mute the lowest string and thus put a really nasty sounding E note on the bottom of the chord)
I think I understand what you are saying...but at the same time, given the example 6stringmerc used, there is nothing to "teach", the student has to practice more. That isn't teachable, the skill is only achieved via practice and work. (I give guitar lesions)
Which we, as a society, also seem reluctant to do. "oh you must not be good at that, try something else", instead of "if you want it you are going to have to work harder"
So, guitar is tough. Because it's not that you can teach someone how to do a G-C chord transition. There's no theory or understanding that goes into it.
It's muscle memory, plain and simple. You just have to do it a lot, over and over. I've had music teachers essentially say "that part sucked, you need to spend more time practicing it". There's no trick to be told, there's no intellectualization that will help. Practice.
It sounds like extremely constructive criticism to me. More specific than anyone has a right to expect; it's exactly what sounded bad, and exactly what needs to be practiced to sound better.