After a lineup show other comedians sometimes ask me for "notes" (feedback) on their performance. I usually can't do this in a useful way, this is specially true for shows I'm helping to run such as our own One Mic Stand-up but also applies when I'm just a member of the line-up.
If I'm at a show, I'm almost always there to work. I may be a performer, I may be the MC, or I may be the producer.
As a performer, the show is divided into three parts. Before I'm on, I'm considering what the audience is like, thinking about my material for the night and trying not to be a bundle of nerves. Then I'm on & when I walk off I'm full of adrenaline for a while. After that I may or may not be paying attention to the show.
As MC, I'm getting everything that applies to me as a performer plus I'm checking that I can see the next performer somewhere in the room & that they know they are on next. I'm watching the audience so I know if I should just bring the next act on or if I need to do some jokes to reset the crowd first. The one person I'm not watching is the comedian. Of course I know they are there, but exactly what they are doing usually doesn't register.
There's a bit more hope when I'm being a producer. Most of the work is before and after the show itself. During the show I'm mainly keeping track of time for the performers and the MC with a small amount of my attention keeping an eye on the lighting and listening to sound quality. It's pretty low energy, I could take notes for performers but they'd need to ask me first. Unless it's someone I haven't seen perform for a while or I know they are doing new stuff, the chances are good that I'm not paying attention to them.
I'm not upset if I'm asked, but I usually can't give anything meaningful. That's just my reality.